Partial Substitution of Chemical N with Solid Cow Manure Improved Soil Ecological Indicators and Crop Yield in a Wheat–Rice Rotation System

: Alternative fertilizers are essential to minimizing the deteriorating effects of chemical fertilizers on soil and water quality/health. Accordingly, the present work investigated the effects of combined organic–inorganic fertilization (COIF) on wheat and rice yields, soil nutrients, and soil Cd accumulation. Hence, seven different treatments were set up: control (CK); conventional fertilization (CF); adequate fertilization (OF); organic fertilizer replacing 25% (T1) and 50% (T2) of OF; and organic nitrogen (N) replacing 25% (M1) and 50% (M2) of OF-N. Overall, significant increases occurred in the yields of COIF crops. Compared with the CF, the highest wheat and rice yields happened in the M1 treatment (with a difference of approximately 18.5%) ( p < 0.05). COIF slightly alleviated soil acidification, and improved the cation exchange capacity (CEC) of the study soils. Furthermore, COIF treatments significantly increased the contents of total phosphorus, total potassium, available phosphorus, and available potassium by 6.35 to 16.9%, 3.17 to 10.9%, 5.53 to 28.7%, and 2.6 to 12%, respectively ( p < 0.05). Nevertheless, negligible increases took place in the Cd content of COIF soils compared with that of the CK. Altogether, our results concluded that 25% replacement of OF-N by organic N (M1) effectively improved the fertility/ecological sustainability of the study soils.


Introduction
Chemical fertilization is a major contribution to large-scale agricultural production in China [1].Over the past few decades, chemical fertilization of Chinese croplands has increased both in the rate and frequency due to rapid population growth, agricultural development, and the increasing demand for food [2].In accordance, China's fertilizer consumption reached about 59.84 million tons by 2016 [3].
Despite the encouraging effects of chemical fertilizers on soil productivity/crop growth, excessive fertilization might cause serious environmental problems, threatening the quality of soil and water [4][5][6][7][8].As such, heavy chemical fertilization can adversely affect soil microbial/enzyme properties/activities, along with causing soil hardening, soil acidification, and nutrient leaching [4].Furthermore, long-term chemical inputs threaten sustainable agricultural development via reducing the nutrients' use efficiency/deteriorating soil fertility, and imposing non-point source pollution [4][5][6][7].For instance, long-term N overfertilization stimulated soil acidification and salinization in greenhouse vegetable soils of Shandong, North China, and Zhejiang, East China, due to the nitrification reactions that lowered the pH [8].High soil acidity and salinity caused by excessive N applications increased the transfer of cadmium (Cd) from the soil to the wheat grains in Anhui Anthrosol, East China [9].Similar fertilization schemes reduced bacterial diversity and altered the bacterial community composition of northeastern Chinese black soils within two successive crop seasons [10].These findings emphasize the necessity of the substitution of chemical fertilizers with organic amendments, which in turn cause less environmental degradation and contribute to soil health and quality [11][12][13].
Accordingly, Gao et al. [11] found that the long-term (16-year) combined application of manure and chemical fertilizers enhanced soil fertility and provided a healthier ecosystem for microbial diversity/population compared with chemical fertilization alone in the Inner Mongolia Autonomous Region, North China.Du et al. [12] also suggested that the partial replacement of chemical fertilizer with manure increased the formation of macro-aggregates, soil carbon storage, and soil productivity in an arid gravel field of Northwest China.Consistently, Shan [13] showed that a 20% reduction in N application to a Zhejiang paddy field, East China, not only maintained crop yield, but also alleviated soil acidification.In a recent review, Wang et al. [14] reviewed the challenges of biochar application for the remediation of heavy-metal-contaminated soils.Amirahmadi et al. [15] concluded on the efficacy of the co-application of compost and chemical fertilizer on crop productivity and ecological qualities in an irrigated dryland winter wheat.Hence, several workers suggested the co-application of chemical N and manure/organic amendments to achieve optimal crop yield and to improve soil physiochemical and biological properties [13,16].
Further, the partial replacement of chemical N with an organic amendment might also minimize the adverse ecological impacts of the sole application of organic or inorganic fertilizers.It is also worth noting that various organic amendments and their substitution rates may have different effects on soil qualities and crop growth, leading to different efficiencies of the latter.This is of particular importance in the large-scale/diverse agricultural production systems that take place in China.Thus, the present field trial was carried out in Feidong County, Anhui Province, East China, to explore the effects of the co-application of chemical N fertilizer and solid cow manure on the following: (1) the yields of wheat and rice and their grains' quality, (2) the total and available contents of soil phosphorus (P) and potassium (K), and (3) soil ecological quality/risk, determined via assessing the total and available contents of Cd in soils.
We hypothesized that the partial replacement of chemical N with solid cow manure would improve soil fertility and crop yield/quality, particularly in long-term trials.We hope that the results of this study provide an alternative/sufficient fertilization scheme for farmers and the stakeholders to maintain/maximize crop productivity/soil fertility and to minimize the bioavailability of toxic metals in farmland soils.

Study Area
Feidong County (117 • 38 ′ 17 ′′ E, 32 • 12 ′ 99 ′′ N), a major agricultural region in Anhui Province, East China, was selected for this study.This area has a subtropical humid monsoon climate with a mean annual temperature and precipitation of 15.5 • C and 879 mm, respectively.The total cultivated land in Feidong County is about 76,700 ha, and the major soil types are paddy soils and yellow cinnamon soils.Table 1 presents the basic physical and chemical properties of the study's paddy soils.A winter wheat-summer rice rotation is the main cropping system in the study area.In this study, wheat (Guohong No. 9) was planted on 10 November 2021 and harvested on 31 May 2022; rice (Fengliang Youxiang No. 1) was planted on 10 July 2022, and harvested on 21 October 2022.
The fertilization schemes were in accordance with the local management practices.Further, disease control management via pesticides and herbicides, irrigation rate/frequency, and the application of other agrochemicals were all consistent with the local operations during both wheat and rice cultivation.Tables 2 and 3 summarize the details of fertilization schemes in different treatments under both wheat and rice cultivations.
The present study employed a randomized complete block design with seven treatments, including control (CK); conventional fertilization scheme (CF) with N, P, and K application rates of 181.5, 112.5, and 112.5 kg ha −1 , respectively, for wheat, corresponding to 150, 67.5, and 67.5 kg ha −1 , respectively, for rice; adequate fertilization (OF) with N, P, and K application rates of 191.4,75, and 78.75 kg ha −1 , respectively, for wheat, corresponding to 210, 60, and 90 kg ha −1 , respectively, for rice; organic fertilizer replacing 25% of OF (T1), with solid cow manure applications of 2815 (wheat) and 3088 kg ha −1 (rice) and replacing 25% of OF-N, -P, and -K; organic fertilizer replacing 50% of OF (T2), with solid cow manure applications of 5629 (wheat) and 6176 (rice) kg ha −1 and replacing 50% of OF-N, -P, and -K contents; organic N replacing 25% of OF-N (M1), with solid cow manure applications of 2815 (wheat) and 3088 kg ha −1 (rice) and replacing 25% of OF-N, while both P and K remained unchanged; organic N replacing 50% of OF-N (M2), with solid cow manure with amounts of 5629 (for wheat) and 6176 (for rice) kg ha −1 and replacing 50% of OF-N, while both P and K remained unchanged (Tables 2 and 3).
All treatments were replicated three times.Overall, 21 experimental plots were set up for this study in October 2021.Each plot had dimensions of 5 m (width) × 6 m (length) (30 m 2 ).The plot ridges were stacked with cement to block/prevent surface runoff.

Plant Sample Collection and Analyses
For each treatment, the number of effective panicles and the grains per panicle were measured/counted in three different replicates within an area of 1 m 2 prior to the harvest.Then, the panicles were harvested at the maturity stage.The collected samples were washed thoroughly with ultrapure water, oven-dried at 105 • C for 30 min, and then dried again to a constant weight at 70 • C. The panicles were then threshed manually, and the grains were air-dried to ~13% moisture content to determine the crop yields.Three samples of 1000 filled grains from each plot were randomly selected, then weighed to determine the 1000-grain weight.
The Kjeldahl method was used to determine the grain protein contents following grain N measurements.In accordance, oven-dried grains were digested with concentrated H 2 SO 4 in the presence of CuSO 4 •5H 2 O-K 2 SO 4 as the catalyst.Then, the concentrations of grain N were measured using an automatic Kjeldahl N analyzer (SKD-800, Shanghai Peiou Analytical Instrument Co., Ltd., Shanghai, China) following the national food safety standard method formulated by China (GB5009. .Finally, the grain protein contents were calculated by multiplying the N contents by the conversion coefficients of 5.7 and 5.95 for wheat and rice, respectively.

Soil Sample Collection and Analyses
At the sites of plant samples, we also collected the topsoil samples (0-20 cm).The collected soil samples were air-dried and sieved (2 and 0.149 mm) for the subsequent measurements.
Soil pH was measured at a soil:water ratio of 1:2.5 using a glass electrode pH meter (Sartorius PB 10, Shanghai, China); soil cation exchange capacity (CEC) was determined via a rapid barium chloride-sulfuric acid method; soil available P (AP) and total P (TP) were determined using molybdenum blue calorimetry (ZENit-700P atomic absorption spectrophotometer) after soil extraction using 0.5M NaHCO 3 and via soil digestion with acid-perchloric acid-sulfuric, respectively; soil available K (AK) and total K (TK) were determined using a flame photometer (fp640 Shanghai INESA Scientific Instruments Co., Ltd., Shanghai, China) after soil extraction with 1N NH 4 OAc (pH:7) and via soil digestion with NaOH followed by 1:1 HCl and 1:3 H 2 SO 4 .A graphite furnace-flame spectrophotometer (iCE 3500 Thermo, Thermo Fisher Scientific Ltd., Waltham, MA, USA) was used to determine the total and available contents of soil Cd after soil extraction via acid digestion with an aqua regia-perchloric acid mixture and leaching with DTPA extractant.

Data Analysis
The mean and the standard deviations of the raw data were calculated using Microsoft Excel 2016.SPSS 22.0 (IBM Corporation, Armonk, NY, USA) was used to analyze the significant differences, correlations, and linear fitting of the data.All the graphs were plotted with Origin 2017C (OriginLab Inc., Northampton, MA, USA).

Crop Yield
The indicators of wheat and rice yields under different treatments are shown in Tables 4 and 5, respectively.Overall, increases happened in the components of both crops with the combined application of organic and inorganic fertilizers (COIFs).For instance, during the wheat season, T1 and M1 treatments significantly increased the effective panicle numbers by 18.7 and 24.1%, respectively, compared with the CF treatment, corresponding to 12.8 and 22.6% for the rice season (p < 0.05).From Tables 4 and 5, higher crop yields happened via the replacement of chemical N by organic N than those under the replacement of chemical fertilizer by organic fertilizer, especially for M1.In accordance, the highest wheat (5200 kg ha −1 ) and rice (7900 kg ha −1 ) grain yields happened in M1, both being 18.5% higher than those under CF treatment (p < 0.05).The highest grains per panicle happened in T2 wheat (40.4) and M1 rice (143), these being 18.8 and 13.5% higher than those under the CF treatment, respectively (p < 0.05).Nevertheless, for both crops, no significant differences occurred among COIF and CF soils in their contents of a 1000-grain weight.

Crop Grain Protein
Figure 1 illustrates the protein contents of crop grains for the different treatments.Considering CF to be the reference, insignificant increases happened in the protein contents of wheat grains as follows: M1 (15.71%) > M2 (13.17%) > T1 (10.39%) > T2 (7.01%), corresponding to significant increases in the protein contents of rice grains in the order of M1 (7.40%) > T1 (4.67%) > T2 (3.85%) > M2 (3.55%).In accordance, higher protein contents occurred at the manure replacement rate of 25% than they did at that of 50%, although the differences were insignificant.

Crop Grain Protein
Figure 1 illustrates the protein contents of crop grains for the different treatments.Considering CF to be the reference, insignificant increases happened in the protein contents of wheat grains as follows: M1 (15.71%) > M2 (13.17%) > T1 (10.39%) > T2 (7.01%), corresponding to significant increases in the protein contents of rice grains in the order of M1 (7.40%) > T1 (4.67%) > T2 (3.85%) > M2 (3.55%).In accordance, higher protein contents occurred at the manure replacement rate of 25% than they did at that of 50%, although the differences were insignificant.

Soil pH and CEC
Figure 2 shows the contents of soil pH and cation exchange capacity (CEC) under different treatments.Apparently, the lowest pH values happened in CF treatments for both wheat (7.91) and rice (8.03) crops.The CK treatments generally had the highest pH values of 8.14 and 8.18 for wheat and rice soils, respectively (Figure 2a).As can also be seen, slight increases of 2 to 2.52% happened in the soil pH of manure-amended treatments compared with the CF treatment during wheat season (Figure 2a), corresponding

Soil pH and CEC
Figure 2 shows the contents of soil pH and cation exchange capacity (CEC) under different treatments.Apparently, the lowest pH values happened in CF treatments for both wheat (7.91) and rice (8.03) crops.The CK treatments generally had the highest pH values of 8.14 and 8.18 for wheat and rice soils, respectively (Figure 2a).As can also be seen, slight increases of 2 to 2.52% happened in the soil pH of manure-amended treatments compared with the CF treatment during wheat season (Figure 2a), corresponding to an increase of 1 to 1.62% for rice (Figure 2a).Unlike the pH values, the lowest and the highest CEC contents took place in CK and manure-amended soils, respectively (Figure 2b).Accordingly, the CEC contents of wheat and rice CK treatments were 20.5 and 20.6 cmol kg −1 , respectively (Figure 2b).Further, the highest CEC contents happened in wheat T2 (24.3 cmol kg −1 ) and rice M2 (25 cmol kg −1 ) soils, these being 15 and 15.7% higher, respectively, than those of the corresponding CF soils (Figure 2b).
Agronomy 2024, 14, x FOR PEER REVIEW 7 of 15 to an increase of 1 to 1.62% for rice (Figure 2a).Unlike the pH values, the lowest and the highest CEC contents took place in CK and manure-amended soils, respectively (Figure 2b).Accordingly, the CEC contents of wheat and rice CK treatments were 20.5 and 20.6 cmol kg −1 , respectively (Figure 2b).Further, the highest CEC contents happened in wheat T2 (24.3 cmol kg −1 ) and rice M2 (25 cmol kg −1 ) soils, these being 15 and 15.7% higher, respectively, than those of the corresponding CF soils (Figure 2b).

Total and Available P in Soil
Figure 3 shows the total and available P contents of soils under different treatments.In accordance, compared with those in the CF treatment, significant increases happened in the TP contents of the T2, M1, and M2 treatments during the wheat season, ranging from 12.5 to 16.9% (p < 0.05) (Figure 3a).Herein, insignificant increases of 4.76 and 6.35% took place in OF and T1 soil TP contents, respectively, compared with those in the CF treatment.Similarly, compared with the CF treatment, significant increases appeared in the TP contents of rice in T1, T2, M1, and M2, ranging from 13.6 to 22.7% (p < 0.05) (Figure 3a).Consistent with the soil TP contents, T2, M1, and M2 treatments significantly increased the wheat soil AP contents, ranging from 16.2 to 28.7%, compared with those in the CF treatment (p < 0.05) (Figure 3b).Similarly, OF and T1 caused insignificant increases in wheat soil AP contents of 5.94 and 5.53%, respectively, compared with the CF treatment.In addition, T1, T2, M1, and M2 treatments significantly increased the AP contents of rice soils by 14.6, 22.2, 27, and 31%, respectively, compared with the CF treatment (p < 0.05) (Figure 3b).

Total and Available P in Soil
Figure 3 shows the total and available P contents of soils under different treatments.In accordance, compared with those in the CF treatment, significant increases happened in the TP contents of the T2, M1, and M2 treatments during the wheat season, ranging from 12.5 to 16.9% (p < 0.05) (Figure 3a).Herein, insignificant increases of 4.76 and 6.35% took place in OF and T1 soil TP contents, respectively, compared with those in the CF treatment.Similarly, compared with the CF treatment, significant increases appeared in the TP contents of rice in T1, T2, M1, and M2, ranging from 13.6 to 22.7% (p < 0.05) (Figure 3a).Consistent with the soil TP contents, T2, M1, and M2 treatments significantly increased the wheat soil AP contents, ranging from 16.2 to 28.7%, compared with those in the CF treatment (p < 0.05) (Figure 3b).Similarly, OF and T1 caused insignificant increases in wheat soil AP contents of 5.94 and 5.53%, respectively, compared with the CF treatment.In addition, T1, T2, M1, and M2 treatments significantly increased the AP contents of rice soils by 14.6, 22.2, 27, and 31%, respectively, compared with the CF treatment (p < 0.05) (Figure 3b).

Total and Available K in Soil
Figure 4 illustrates the contents of soil total K (TK) and available K (AK) under ferent treatments.As can be seen, significant increases happened in the TK contents un the T2, M1, and M2 treatments during the wheat season, of 7.56, 5.24, and 10.9%, resp tively, compared with those under the CF treatment (p < 0.05) (Figure 4a).Herein, OF a T1 caused insignificant increases in TK of 1.03 and 3.17%, respectively, compared w those under the CF treatment (p < 0.05) (Figure 4a).It is also worth mentioning that wh under the M2, M1, and T2 treatments had significant differences in its TK contents ( 0.05).For the rice season, compared with the CF treatment, the TK contents under the T2, M1, and M2 treatments significantly increased by 5.43, 8.74, 10.4, and 14.4%, resp tively (p < 0.05) (Figure 4a).Consistent with the TK results, significant increases occur in soil available K (AK) contents in the T2, M1, and M2 treatments during the wheat sea of 7.30, 5.22, and 12%, respectively, compared with that under the CF treatment (p < 0 (Figure 4b).Herein, T1 caused an insignificant increase in the AK content of 2.6%.Co spondingly, compared with the CF treatment, the AK contents of rice under the T1, M1, and M2 treatments significantly increased by 6.77, 11.2, 10.8, and 16.1%, respectiv (p < 0.05) (Figure 4b).

Total and Available K in Soil
Figure 4 illustrates the contents of soil total K (TK) and available K (AK) under different treatments.As can be seen, significant increases happened in the TK contents under the T2, M1, and M2 treatments during the wheat season, of 7.56, 5.24, and 10.9%, respectively, compared with those under the CF treatment (p < 0.05) (Figure 4a).Herein, OF and T1 caused insignificant increases in TK of 1.03 and 3.17%, respectively, compared with those under the CF treatment (p < 0.05) (Figure 4a).It is also worth mentioning that wheat under the M2, M1, and T2 treatments had significant differences in its TK contents (p < 0.05).For the rice season, compared with the CF treatment, the TK contents under the T1, T2, M1, and M2 treatments significantly increased by 5.43, 8.74, 10.4, and 14.4%, respectively (p < 0.05) (Figure 4a).Consistent with the TK results, significant increases occurred in soil available K (AK) contents in the T2, M1, and M2 treatments during the wheat season of 7.30, 5.22, and 12%, respectively, compared with that under the CF treatment (p < 0.05) (Figure 4b).Herein, T1 caused an insignificant increase in the AK content of 2.6%.Correspondingly, compared with the CF treatment, the AK contents of rice under the T1, T2, M1, and M2 treatments significantly increased by 6.77, 11.2, 10.8, and 16.1%, respectively (p < 0.05) (Figure 4b).

Total and Available K in Soil
Figure 4 illustrates the contents of soil total K (TK) and available K (AK) under ferent treatments.As can be seen, significant increases happened in the TK contents un the T2, M1, and M2 treatments during the wheat season, of 7.56, 5.24, and 10.9%, resp tively, compared with those under the CF treatment (p < 0.05) (Figure 4a).Herein, OF T1 caused insignificant increases in TK of 1.03 and 3.17%, respectively, compared w those under the CF treatment (p < 0.05) (Figure 4a).It is also worth mentioning that wh

Soil Ecological Risk
Figure 5 illustrates the total and available contents of Cd in soils under different treatments.In accordance, slight increases happened in the soil total Cd contents of both wheat and rice in manure-amended soils compared with those in the non-manure amended soils.Compared with the CF treatment, the total Cd contents of wheat in the T1, T2, M1, and M2 soils increased by 2.43, 3.89, 3.41, and 4.37%, respectively (Figure 5a), corresponding to increases of 0.93, 2.09, 1.63, and 2.56%, respectively, for the rice season (Figure 5a).Similar to the total Cd contents, negligible increases occurred in topsoil Cd availability with increasing rates of manure replacement (Figure 5b).Accordingly, compared with the CF treatment, the DTPA-Cd contents of wheat in the T1, T2, M1, and M2 soils increased by 1.59, 2.03, 2.46, and 3.47%, respectively (Figure 5b), corresponding to increases of 0.85, 1.28, 1.42, and 1.99%, respectively, during the rice season (Figure 5b).

Soil Ecological Risk
Figure 5 illustrates the total and available contents of Cd in soils under different treatments.In accordance, slight increases happened in the soil total Cd contents of both wheat and rice in manure-amended soils compared with those in the non-manure amended soils.Compared with the CF treatment, the total Cd contents of wheat in the T1, T2, M1, and M2 soils increased by 2.43, 3.89, 3.41, and 4.37%, respectively (Figure 5a), corresponding to increases of 0.93, 2.09, 1.63, and 2.56%, respectively, for the rice season (Figure 5a).Similar to the total Cd contents, negligible increases occurred in topsoil Cd availability with increasing rates of manure replacement (Figure 5b).Accordingly, compared with the CF treatment, the DTPA-Cd contents of wheat in the T1, T2, M1, and M2 soils increased by 1.59, 2.03, 2.46, and 3.47%, respectively (Figure 5b), corresponding to increases of 0.85, 1.28, 1.42, and 1.99%, respectively, during the rice season (Figure 5b).

Crop Yield
Our findings in regard to the enhancing effects of COIF on crop yields are in line with the observations of Kalkhajeh et al. [18], who found increases in the components of rice yield with the co-application of straw and chemical N. Others suggested that an appropriate combination of organic and inorganic fertilizers increases crop yield via increasing the number of spikelets and improving the photosynthetic characteristics [19,20].
Manure application can improve soil organic matter/fertility, as well as soil physiochemical and biological properties, with subsequent increases in soil TN [20].Higher effective panicle numbers and grain yields in ~25% manure replacement compared with ~50% replacement [21] might be attributed to the relatively release of less inorganic nutrients with increasing rates of organic fertilization in T2 and M2 treatments [22].This can lead to insufficient soil nutrient supply during the early tillering and elongation of both crops, resulting in late ripening and declining the yield.Nutrients in chemical fertilizers are readily available for the plants [23].

Crop Yield
Our findings in regard to the enhancing effects of COIF on crop yields are in line with the observations of Kalkhajeh et al. [18], who found increases in the components of rice yield with the co-application of straw and chemical N. Others suggested that an appropriate combination of organic and inorganic fertilizers increases crop yield via increasing the number of spikelets and improving the photosynthetic characteristics [19,20].
Manure application can improve soil organic matter/fertility, as well as soil physiochemical and biological properties, with subsequent increases in soil TN [20].Higher effective panicle numbers and grain yields in ~25% manure replacement compared with ~50% replacement [21] might be attributed to the relatively release of less inorganic nutrients with increasing rates of organic fertilization in T2 and M2 treatments [22].This can lead to insufficient soil nutrient supply during the early tillering and elongation of both crops, resulting in late ripening and declining the yield.Nutrients in chemical fertilizers are readily available for the plants [23].
Nevertheless, no significant differences happened in the 1000-grain weight among the different treatments, which might be attributed to the short duration of this experiment [18].

Crop Grain Protein
Previous works suggested that organic fertilizers can improve the removal/absorption of N and K by crops, explaining the higher protein contents in manure amended crops' grains [7].Others found that reductions happened in the grain protein content of crops following the replacement of inorganic fertilizer with manure [24,25].These contradictory results might be explained by the variations in N supply and removal.For instance, in this study, the highest protein contents took place in the M1 treatment for both wheat and rice grains.This might be attributed to the optimum N availability, which, in turn, has an encouraging effect on protein production by crop grains [26].This is in line with the results of Qu et al. [27], who found increases in the protein content of wheat grains via 25% or 30% replacement of inorganic fertilizer with manure.
It is also worth mentioning that the increasing trend of the protein contents of manureamended grains was consistent with the indicators of wheat yield (e.g., grain yield and effective panicle number).

Soil pH and CEC
Overall, our results revealed slight increases in both soil pH and CEC contents after the partial replacement of chemical fertilizer with solid cow manure.Excessive N application via CF might lead to nitrification reactions, resulting in lower base saturation/alkalinity and subsequently reducing soil pH [8].The replacement of organic fertilizers can effectively inhibit soil acidification via increasing the soil pH buffering capacity [28].The latter happens via the addition of base ions [29], the ammonification of organic N, the decarboxylation of organic anions consuming protons to release CO 2 during OM decomposition [30], and the formation of organic-aluminum complexes that reduce the effective aluminum content in the soil solution.
However, negligible increases in soil pH might be attributed to the short duration of this field trial, and significant encouraging effects can be obtained via longer manure application.Soil CEC refers to the total amount of various cations that can be adsorbed by the soil colloids.It is an important basis for evaluating soil fertility capacity, and for evaluating soil environmental capacity and or pollutant migration and transformation [31].Apparently, higher CEC values happened with increasing rates of manure replacement, corresponding to the increases in soil pH values (Figure 2) [32].Furthermore, manure application enhances soil organic matter.The latter increases soil organic C stock, and finally soil CEC.

Total and Available P and K in Soil
Our resulted revealed that an increase in the proportion of manure application caused a higher increase in soil TP contents for both crops (p < 0.05) (Figure 3a,b), which is consistent with the findings of Yang et al. [33].These results can be explained by the higher P sorption capacity of manure-amended soils, increasing soil TP contents [34,35].Likewise, the higher TP contents in T1, T2, M1, and M2 soils than those in OF soils might also be attributed to their higher P sorption capacity brought on via manure application.
Similar observations happened in terms of the contents of soil AP contents among the different treatments (p < 0.05) (Figure 3a,b).This is in line with the results of Jing et al. [36], who found that changes in AP contents are consistent with those in TP, and both increased with an increasing proportion of organic fertilizers.However, organic amendments increase soil OM content.The latter increases the soil labile P pool and P dissolution, increasing soil P availability [37].Organic fertilization also stimulates the activities of soil microorganisms and phosphatase, accelerating the rate of mineralization [38].Moreover, organic fertilization, particularly in the long term, can improve soil physiochemical properties, which in turn contribute to/stimulate the mineralization of organic matter in the soil [39,40].
Our observations of the significant effects of COIF treatments on TK contents in both wheat and rice soils were in agreement with the findings of Nguyen et al. [41] for CF (p < 0.05) (Figure 4a,b) because the co-application of organic and inorganic fertilizers causes a higher rate of K conversion than does chemical fertilization alone [42].This can also explain the increases in TK content that took place with increasing rates of manure replacement [43].Likewise, higher increases occurred in the AK contents of manureamended rice and wheat soils compared with that under the CF treatment (p < 0.05) (Figure 4a,b).Previous works have suggested that the recycling of organic K is the major pathway for plant K.This explains the increased AK with the increasing rates of manure replacement [44,45].Similar observations happened in the study by Balík et al. [46].In addition, increases in organic fertilization stimulate/improve the microbial conversion of organic K into inorganic K.

Soil Ecological Risk
Our results suggest slight increases in soil total and available Cd contents with increasing rates of manure replacement.Previous studies have shown that organic fertilizers, particularly livestock and poultry manures, are the major sources of heavy metals including Cd [47,48].Hence, long-term continuous manure application might increase the Cd content in soil, posing potential ecological risks.Furthermore, the higher Cd content of the manureamended treatments might also be attributed to the high buffering capacity of the study soils [49,50].Similar observations were reported by Wang et al. [51] and Guan et al. [52].Nevertheless, the ecological risk of combined organic-inorganic fertilization (COIF) was small due to the slight increases in Cd accumulation in COIF-amended soils.

Environmental Implications
Taking into account the current fertilization schemes that are practiced in typical wheat-rice rotation systems in China, it is essential to optimize/reduce the rate of chemical fertilization.Our findings revealed that partial replacement of chemical fertilizers with different rates of solid cow manure, as alternative strategies, not only improved crop productivity but also enhanced the contents of total and available P and K in the study soils.Apparently, large inputs of readily available N and P via chemical fertilization lead to their excessive accumulation in the soil, increasing potential surface/subsurface leaching to the adjusting water bodies, degrading water quality via eutrophication [53,54].This, in particular, can be stimulated via the anaerobic conditions during rice cultivation, which in turn enhance P dissolution via a reduction in metal oxides/hydroxides [55].However, the effect of soil acidification on soil P mobilization can be partially neutralized via manure application [28,29].In addition, manure application/organic amendments increase the contents of base cations in agricultural soils [29].The latter contribute to the larger soil P-holding capacity and P availability for plant removal/growth [53].Likewise, manure fertilization can also improve soil aggregation, which in turn increases the soil water-holding capacity, resulting in a lower irrigation water volume and frequency.Further, manure application reinforces the mutual relationship between soil aggregation and organic C storage.In other words, manure-driven organic C is a major cementation material that binds the smaller soil particles to form soil aggregates.Additionally, soil aggregates are the major storage sites for the accumulation of organic C fractions, protecting them against microbial decomposition [56,57].Nevertheless, manure application slightly increased soil total and available Cd, threatening soil quality/health and food safety.This effect can be triggered via longer manure fertilization of the study paddy soils.Hence, practical pre-treatment measures should be taken into consideration to eliminate Cd and other toxic metals from solid cow manure prior to its field application.

Future Research Needs
Despite the significant findings of this study with respect to the effects of the partial replacement of chemical fertilizer with solid cow manure on soil fertility and health/quality, the following issues still warrant further attention:

•
Particle size distribution and chemical composition are the two major factors of animal manures affecting the soil holding capacities of different nutrients.Hence, future works should pay special attention to the effects of these two parameters on soil CEC both for top and subsoil layers.This, in particular, calls for long-term in situ trials for the determining the microbial decomposition of manure.
• Since exchangeable base cations play critical roles in soil P-holding properties, it is essential to determine/document the base cations along with the rate of manure application and their relationship with soil P precipitation and mobilization [34,53].

•
Despite the good documentation of the effects of manure application on P accumulation in the study's COIF-amended paddy soils (0-20 cm), future research should investigate the spatiotemporal subsurface leaching of both organic/inorganic and dissolved/particulate soil P in relation to the long term and the rate of soil cow manure application [58].

•
It is of major importance to study the effects of the co-application of manure and chemical fertilizers on crop yield qualities in longer periods of time [18].

Conclusions
The present field trial concludes on the effects of combined organic-inorganic fertilization (COIF) on crop quality, soil fertility, and ecological risk.Our results revealed that COIF caused significant increases in the yields of wheat and rice compared with the conventional and adequate fertilization schemes.Crop yields increased with increasing substitution rates of chemical fertilizer with solid cow manure, and the highest yield happened at a manure replacement rate of 25%.Furthermore, crop quality components were higher in COIF treatments than in other fertilization schemes.For instance, higher protein contents occurred at manure replacement rates of 25% than 50%.In addition, increases took place in soil CEC contents with increasing rates of manure application.Likewise, higher contents of total and available P and K happened in COIF soils than in other soils.Herein, the replacing effect of organic N was higher than that of manure replacing chemical fertilizers.Conversely, insignificant increases happened in soil total and available Cd contents of manure-amended soils, indicating the negligible ecological risk of COIF fertilization schemes.However, future works should investigate the long-term spatiotemporal changes in soil qualities, the accumulation/mobilization of soil nutrients, and crop yield indicators in relation to the physiochemical properties of the animal manures/organic amendments used.

Figure 1 .
Figure 1.Protein contents of wheat (a) and rice (b) grains under different treatments.Values are the means with standard deviations shown by vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N

Figure 1 .
Figure 1.Protein contents of wheat (a) and rice (b) grains under different treatments.Values are the means with standard deviations shown by vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 2 .
Figure 2. Soil pH (a) and CEC (b) contents in wheat and rice seasons under different treatments.Values are the means with standard deviations, shown as vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 2 .
Figure 2. Soil pH (a) and CEC (b) contents in wheat and rice seasons under different treatments.Values are the means with standard deviations, shown as vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 3 .
Figure 3. Soil TP (a) and AP (b) contents in wheat and rice seasons under different treatments.Va are means with standard deviations, shown as vertical bars (n = 3).Columns followed by diffe lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilizat OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replac 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 4 .Figure 3 .
Figure 4. Soil TK (a) and AK (b) contents in wheat and rice seasons under different treatment c ditions.Values are means with standard deviations, shown as vertical bars (n = 3).Columns follow by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventio

Figure 3 .
Figure 3. Soil TP (a) and AP (b) contents in wheat and rice seasons under different treatments.Va are means with standard deviations, shown as vertical bars (n = 3).Columns followed by diffe lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilizat OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer repla 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 4 .
Figure 4. Soil TK (a) and AK (b) contents in wheat and rice seasons under different treatment conditions.Values are means with standard deviations, shown as vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 5 .
Figure 5.Total Cd (a) and DTPA-Cd (b) contents in wheat and rice seasons under different treatment conditions.Values are means with standard deviations, shown as vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Figure 5 .
Figure 5.Total Cd (a) and DTPA-Cd (b) contents in wheat and rice seasons under different treatment conditions.Values are means with standard deviations, shown as vertical bars (n = 3).Columns followed by different lowercase letters are significantly different at p < 0.05.CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.

Table 1 .
Physical and chemical properties of Feidong paddy soils.

Table 2 .
Fertilizer types and dosages for different treatments in the wheat season (kg ha −1 ).

Table 3 .
Fertilizer types and dosages for different treatments in the rice season (kg ha −1 ).

Table 4 .
The effects of different replacement rates of organic fertilizer on wheat yield.
CK: control; CF: conventional fertilization; OF: adequate fertilization; T1: organic fertilizer replacing 25% of OF; T2: organic fertilizer replacing 50% of OF; M1: organic N replacing 25% of OF-N; M2: organic N replacing 50% of OF-N.The data are given as average ± standard deviation.Numbers followed by different lowercase letters in the same column are significantly different at p < 0.05.

Table 5 .
The effects of different replacement rates of organic fertilizer on rice yield.The data are given as average ± standard deviation.Numbers followed by different lowercase letters in the same column are significantly different at p < 0.05.