A Field Experiment on Enhancement of Crop Yield by Rice Straw and Corn Stalk-Derived Biochar in Northern China

Biochar, a green way to deal with burning and burying biomass, has attracted more attention in recent years. To fill the gap of the effects of different biochar on crop yield in Northern China, the first field experiment was conducted in farmland located in Hebei Province. Biochars derived from two kinds of feedstocks (rice straw and corn stalk) were added into an Inceptisols area with different dosages (1 ton/ha, 2 ton/ha or 4 ton/ha) in April 2014. The crop yields were collected for corn, peanut, and sweet potato during one crop season from spring to autumn 2014, and the wheat from winter 2014 to summer 2015, respectively. The results showed biochar amendment could enhance yields, and biochar from rice straw showed a more positive effect on the yield of corn, peanut, and winter wheat than corn stalk biochar. The dosage of biochar of 2 ton/ha or 1 ton/ha could enhance the yield by 5%–15% and biochar of 4 ton/ha could increase the yield by about 20%. The properties of N/P/K, CEC, and pH of soils amended with biochar were not changed, while biochar effects could be related to improvement of soil water content.


Introduction
Burning straw in air brings many environmental problems by emitting inhalable particles and gaseous pollutant.It is now one of the main origins of the air pollution in China.In a system of combustion simulation-dilution tunnel sampling, crop straw flaming emitted 7.2-39.0g/kg of PM2.5 (particulate matter size less than 2.5 μm), and in a smoldering combustion situation the PM2.5 emission was even 2.4-11.5 times higher [1].In 2006, straw burning caused 2.17 Mt PM2.5 with a mass of gaseous organic and greenhouse gas pollutant [2].In China, more than 60% of total PAHs were estimated from incineration of biomass and agricultural straw reached 1/3 of incinerating biomass [3].In addition to direct burning, the other way to deal with straw is returning it back to the fields.Although such treatment could promote soil properties and enhance crop productivity [4], the respiration of soil was also stimulated.Zhao et al. [5] investigated that soil respiration could be affected by tillage and crop residue management.Deep moldboard plough and crop residue retained increased soil respiration by 41.9% and 21.0% during winter wheat season and summer maize season, respectively.
Different from direct burning or returning biomass residues to farmland, scientists proposed another way to make biomass charcoal, also called biochar, to be a soil enhancer.Biochar is a product of carbonized biomass during pyrolysis, which was first found in the Central Amazon basin [6].Now biochar is considered more as an effective carbon sequestration material.In Lehmann et al.'s prediction models, emission of CO2 reduced by about 20% with black carbon retained in soil over 100 years.Okimori et al. [7] also suggested that biomass waste could reduce CO2 emission.In soil science studies, researchers focus on the potential of biochar on the enhancement of crop yield [8,9] and biochar-soil, biochar-soil biota interactions [10].
Recent researches showed that biochar amendment could increase water-holding capacity of soil as well as nutrient-holding ability because of its developed porosity structure, high specific surface area, and CEC.Additionally, the characteristics of soil and the climate of the planting area, crop productivity is also related to feedstock, pyrolysis conditions, and dosage of biochar addition [11].This is the reason why researchers obtained different results in some field experiments.For example, positive results suggested biochar amendment improved crop yield [12][13][14][15].Baronti et al. [16] found that yields increased both in wheat fields in Central Italy and maize fields in Northern Italy when coppiced woodlands-derived biochar was added in soils at the rate of 10 ton/ha.In Indonesia, the addition of biochar increased the yield of plants [17].Olmo et al. [18] investigated that olive-tree pruning-derived biochar amendment was related to a higher yield but the nutrient content was not obviously affected.The study in Kaoma, Zambia, Africa showed that maize cob-derived biochar dramatically increased maize yield by over 100% in different soils [19].In Australia, the types and dosages of fertilizer and the existence of microbes could influence the effect of oil mallee charcoal on wheat yield [20].
On the contrary, some researchers suggested that the biochar-amended soil did not promote plant yields, even decreased the productivity at a higher dosage or with some different feedstock, neither in pot nor site experiment [16,[21][22][23][24][25].For example, Rajkovich et al. [26] carried out a greenhouse pot trial and suggested that animal manure biochar and food waste biochar decreased the yield of corn at a high dosage (7%), while lower rates (2%, 0.5%) of biochar could increase the yield.Cornelissen et al. [19] also showed that in Mkushi and the other two experiments in Zambia, neither maize cob nor wood biochar affected the maize yields.Jones et al. [27] conducted a three-year field experiment and found that commercial wood chip biochar affected little on maize yield in the first year and on grass yield in the second year, but enhanced grass yield in the third year.Similar result from the study of Major et al. [28] showed that maize yields were increased by commercial wood biochar in the second year.
In the past five years, scientists in China had paid more attention to biochar use in agriculture soils.Liu et al. [29] suggested that rapeseed and sweet potato yields were increased by 36.02% and 53.77%, respectively, if wheat straw biochar was added into soil at a dosage of 40 ton/ha in Jiangxi Province in Southern China.Zhang et al. [30] concluded that wheat straw biochar could enhance rice yield both in fertilized and unfertilized paddy in Tai Lake plain.Liu et al. [31] conducted a five-crop-season field experiment and found that wheat straw biochar could enhance the crop yield in Henan Province, Central Great Plain of China.
All of reported data in China at present were mostly in Southern China where acidic ultisols is distributed.Nevertheless, the soils and climates in Northern China are quite different from that in Southern China.Herein, to fill the gap of the effect of corn stalk and rice straw-derived biochar on crop yields in Northern China, we performed a field experiment in Hebei Province.

Soil Type and Climate of the Study Area
The field experiment was started from April 2014 in agricultural lands in Fengnan County, Tangshan City, Hebei Province (39°30′47″ N, 118°15′38″ E, Northern China), located in a warm temperate continental monsoon climate region (Figure 1).The soil type in the study area is an inceptisol, based on U.S. Department of Agriculture.According to literature reports [32], during years 1961-2011, the annual rainfall was of 600 mm-700 mm.The precipitation was 500 mm-600 mm in 2014 and average temperature of the year 2014 was 12 °C-14 °C.The above data was recorded from reports of the Hebei Province Meteorological Bureau.

Biochar
Corn and rice are the main crops in the study area.We chose these two abundant biomass, corn stalk and rice straw, to make biochar in a kiln near the farmland, which was used to produce commercial charcoal with wood chips.It could create an air-limited condition so that the feedstocks could slowly pyrolyze.The pyrolysis temperature was about 450 ± 50 °C.A whole pyrolysis progress lasted one week with two tons of biochar product.The biochar derived from corn stalk and rice straw were characterized using an elemental analyzer (EA-3000, Euro VECTOR) as listed in Table 1.

Field Experiment
The site field was divided into three parts for planting peanuts (P), sweet potatoes (S) and corn (C).Biochar from different feedstock was added into soils at the time of sowing with dosages (1 ton/ha, 2 ton/ha or 4 ton/ha) in ridges with blank column, as shown in Figure 2. The width of each column was about 1 m, and the length of each column depended on practical field conditions.Herein, corn and sweet potato columns were about 100 m in length, while peanut columns were about 83 m length.After the rain on 26 April 2014, corn was sown two days later.The routine cultivates with compound fertilizer (N/P/K) and acetochlor (50%) was added at the rate of 750 kg/ha and 2-3 L/ha, respectively.Peanut was sown two days after corn with the same treatment and furthermore, 3 L/ha of atrazine (20%) was added on 22 June according to growth situation of crop in the fields.Sweet potatoes were planted on 7 May after biochar and compound fertilizer (N/P/K, 450 kg/ha) were added.With five months of common farming practicing, crops were harvested in September to October 2014 (peanut on 6 September, sweet potato on 8 October), then the yield data was collected in each condition.In spring season tests, the yield was harvested as a lump without replication, while in winter, each condition was replicated in triplicate to collect the yield data.

Soil Sampling and Analysis
Soil samples were taken from the topsoil with 0 cm-15 cm depth on 26 July 2014, after about two months following biochar addition.The samples points were shown in Figure 2.
The soil samples were dried at room temperature and ground to less than 1 mm.Soil pH was measured in 1:2.5 soil/DI water.The soil pH is around 7. Fractions of heavy metals were measured by BCR sequential extraction method [33] and analyzed by ICP-OES (Spectro Blue, Germany).The cation exchange capacity (CEC) was determined by ammonium acetate extract method.
Total N was analyzed following the Kjeldahl procedure (NY/T 53-1987, national standard method).Total P was analyzed following the method NY/T 83-1988.Total K was analyzed following the method NY/T 87-1988.
Two kinds of biochar were mixed with soil in the lab at different rates (1%, 2%, and 10%) to measure the soil water-holding capacity (WHC).WHC was measured according to the method by Outi Priha [34].Independent t-test was used to confirm if different treatments affect WHC by GraphPad Prism version 6.00 for Mac OS X (GraphPad Software, La Jolla, CA, USA).

Biochar Effects on Crop Yield
The crop yields in the field experiments were collected and are displayed in Table 2.The data showed that biochar addition could enhance the crop yields.The yield of the corn on the control soils without biochar weighed 0.5 ton/ha.Obviously, corn stalk-derived biochar (CB) increased the corn yield to 12.18 ton/ha and 12.6 ton/ha by the dosage of 2 ton/ha and 4 ton/ha biochar adding, respectively.Similarly, rice straw-derived biochar (RB) increased the corn yield to 12.36 ton/ha and 12.96 ton/ha by the dosage of 2 ton/ha and 4 ton/ha, respectively.In comparison to the corn yield of 4.2 ton/ha without biochar amendment, CB enhanced the peanut yield to 4.68 ton/ha and 5.1 ton/ha at the dosage of 2 ton/ha and 4 ton/ha, respectively.Likewise, 2 ton/ha and 4 ton/ha RB raised the peanut yield to 4.98 ton/ha and 5.22 ton/ha, respectively.Interestingly, similar to the corn yield, RB could enhance more peanut yield than CB.In addition, the sweet potato yield was also affected by adding biochar.For example, with 2 ton/ha RB addition, sweet potato yield was 37.62 ton/ha and with 4 ton/ha biochar that was 38.94 ton/ha, while without biochar the yield was only 33 ton/ha.Furthermore, compared with the effect of CB on the corn and the peanut, CB affected much more on sweet potato yield.Even at a dosage of 1 ton/ha, CB could improve sweet potato yield to 39.6 ton/ha.
For winter wheat, the yield was 8.6 ± 1.25 and 6.9 ± 2.01 ton/ha with CB addition at rate of 2 ton/ha and 4 ton/ha, respectively.Whereas RB increased the yield from 7.7 ± 0.09 ton/ha to 9.0 ± 0.55 ton/ha with the biochar of 2 ton/ha to 4 ton/ha.According to above productivity harvest, it is obvious that higher dosage of both kinds of biochar has a more positive effect on crop yields.Compared with other field scale experiments about the effect of biochar application on crop yields (Table 3), of which most suggested that the addition enhanced the yield, the rate of increase in our study was about 11%-25% higher than most studies, except for the yield that was increased by over 100% than that reported by Yamato et al. [17].According to the soil taxonomy map from U.S. Dept. of Agriculture, the soil in the study of Manuel Olmo et al. [18] was similar to the soil in our study and the results were similar that biochar increased the yield by about 20%.

Biochar Effects on N/P/K, CEC, and pH of Soils
In our study area, even though the addition of biochar enhanced the yields of crops, it did not affect on the total content of N/P/K in soils (Table 4).Likewise, cation exchange capacity (CEC), as an important index to evaluate soil fertility, was not much changed by biochar in this case (Table 4).The similar results were also investigated in the study of Borchard et al. [21].However, more studies investigated that biochar could enhance the soil nutrient stocks [15,16].From Table 4, the value of pH was somehow decreased a little by the biochar addition, especially in peanut-planted soil.However, to some extent, we suggested that the pH of soils was not changed after biochar amendment in such a period of time.It is known that water plays an important role in Northern China because of the limit rainfall there.Thus, we consider that biochar application may enhance crop yield by holding more water in the soil and the WHC results in our lab experiment showed that biochar addition could increase soil water content, especially at a high rate (10%) of application.Change of soil WHC with biochar addition at rates of 1% and 2%, which were similar to the application in the field in our study, seemed to not be significant (statistical test), but as it has been reported by Sun et al. in 2014 [46], straw biochar could significantly increase the available water content of soils.Kristiina Karhu mentioned that biochar application could increase WHC by 11% [47].Ammal Abukari also suggested that biochar addition increased soil moisture content [41].For this aspect, we will keep the study and further investigate whether the biochar can enhance the soil moisture content and water uptake.

Conclusions
The first field experiment of biochar application in Hebei Province, Northern China, focused on the effects of rice straw and corn stalk-derived biochar on the crop yields of corn, peanut, and sweet potato during one crop season from spring to autumn 2014, and wheat from winter 2014 to summer 2015, respectively.The yield in the first season could be measured after biochar amendment and rice straw-derived biochar showed a more positive effect on the yield of corn, peanut, and winter wheat than corn stalk biochar.A lower dosage of biochar (2 ton/ha or 1 ton/ha) could enhance yields by 5%-15% and biochar of 4 ton/ha could increase yields by about 20%.The properties of N/P/K, CEC, and pH of soils were not changed before and after biochar addition and we consider such yield enhancement of biochar effects could be related to improvement of soil water content rather than elements.Further investigation on this study area is on the way.Due to the present results, further investigation will focus on the biochar effect on physical and chemical properties of the soil and the mechanism of crop yield variation.

Figure 1 .
Figure 1.The location of the study area.

Figure 2 .
Figure 2. Sketch map of the field experiment design and soil sampling sites.

Figure 3 .
Figure 3. Snapshot of sweet potato on 26 July 2014 (a) and winter wheat on 10 October 2014 (b).

Table 1 .
Analyses of biochar derived from corn stalk and rice straw.

Table 2 .
Yields of different crops on biochar amended soil *.

Type and Sowing Time to Harvest Time Crop Yield (ton/ha) Crop Yield with Corn Stalk Derived Biochar (CB) (ton/ha) Crop Yield with Rice Straw Derived Biochar (RB) (ton/ha) No biochar 1 ton/ha CB 2 ton/ha CB 4 ton/ha CB 2 ton/ha RB 4 ton/ha RB
* Only the sum of yield data without replication (April-October 2014) is shown; For tests with replication in triplicate (October 2014-June 2015), the data showed the mean and standard deviation values of the yield.

Table 3 .
Summary of the field experiments on crop yield by biochar amendment.

Table 4 .
The determination of N/P/K, CEC, and pH of soils.