1. Introduction
Abandoned farmland is widespread over the world [
1,
2,
3], in particular in arid and semi-arid regions. Primary causes of abandoned farmland in arid and semi-arid zones are soil fertility decline and soil secondary salinization. Abandoned farmland will gradually lose its soil nutrients [
4], suffer degradation of soil physical characteristics [
5], and eventually result in desertification [
6]. Reclamation of abandoned farmland is an important practice to protect agricultural ecological systems, and is vital to a sustainable agriculture and a viable food security for a rapidly increasing world population.
Artificial reclamation, which involves efficient irrigation and fertilization, is more effective than natural recovery of an abandoned farmland. A process of vegetation recovery through natural succession will take much longer [
7]. However, the effect of soil fertility improvement and soil salinization control using different artificial measures is significantly different [
8,
9,
10]. For instance, different artificial vegetation recovery treatments will have a varied effect on the improvement of soil properties, such as soil nutrient, microbial biomass and enzyme activity [
11,
12]. Therefore, the study to assess different artificial reclamation treatments in a region will help with the selection of a proper reclamation mode for abandoned farmland.
The ecological effects of vegetation on soil nutrient content, such as carbon (C), nitrogen (N) and phosphorus (P), during the reclamation of abandoned farmland have drawn the attention of many researchers [
13]. Studies have shown significant interactions between soil and vegetation during a reclamation process, such that soil and vegetation are constantly evolving and developing [
14,
15]. As vegetation recovers, soil nutrient contents will increase. These changes promote further recovery of vegetation.
Soil microorganisms drive soil structure formation, soil organic matter decomposition and soil nutrient transportation to plants. Therefore, it is essential to understand soil microbial sensitivity to environmental changes, including soil salinization and land reclamation. Saline stress may have a detrimental influence on microbial soil communities and their activities [
16,
17,
18]. Since soil microbial biomass carbon (SMBC) and soil microbial nitrogen (SMBN) in microbial communities are important components of soil nutrient pools, changes of SMBC and SMBN may serve as pronounced indicators in soil microorganisms and may help to recognize the reclamation effect of abandoned farmland [
19,
20].
Soil enzymes are critical in soil biochemical transformations. Enzyme activity, the quantity of active enzyme present, is useful in assessing soil quality of reclamation land [
21,
22,
23]. Soil enzymes come from living and dead microbes, plant roots and residues, and soil animals. They play an important role in maintaining healthy soil ecological conditions, such as both physical and chemical properties and fertility [
24]. Soil enzyme activities catalyze several vital reactions in soils, such as decomposition of organic matter, formation of agglomerates and nutrient cycling [
25,
26,
27]. They are important indicators for evaluating soil fertility levels in different reclamation treatments.
Oasis agriculture has developed rapidly in Xinjiang during the past 60 years. This region is now an important grain and cotton production area of China. As a marginal region with a fragile ecological balance, irrigation, fertilization and other human activities altered regional soil properties [
28]. In particular, the irrational use of land and water resources has caused secondary salinization of farmland, such as intensive irrigation, overuse of fertilizers and inappropriate planting patterns. More than one-third of the total farmland was abandoned in Xinjiang [
29]. With the wide-spread adoption of drip irrigation technology since 1996, large areas of farmland have been reclaimed in the region. However, there is a debate about which fit-for-purpose reclamation method should be adopted. Little is known about the influence of these methods on soil quality. In this study, a field experiment of four artificial reclamation treatments and one natural succession was designed to explore the best reclamation mode by assessing and comparing the effects of different reclamation treatments in abandoned farmland in Xinjiang. There are two major steps in this study: (1) analysis of the difference in soil nutrient, microorganism and enzyme activity under different reclamation treatments; (2) a comprehensive assessment on the most optimal reclamation treatment. The outcome of this study will provide a much needed reference framework for future studies of reclamation in abandoned farmland, adjustment of cropping pattern and sustainable utilization of salinized farmland.
4. Discussion
The results of this study indicate that four artificial reclamation treatments significantly altered the physicochemical properties of abandoned farmland (
Figure 3). Soil salinity, which is the major concern in this region, declined most in CC treatment. Cotton could remove between 80 and 120 kg NaCl ha
−1 from the soil during a growing season [
34]. If the soluble salt content of the soil was 0.6% (moderate salinization), there would be 15,000 kg salt ha
−1 in the 0–20 cm soil depth. This means the uptake of salt by cotton would account for a very small part of the soluble salt content in the soil. It was more likely that soluble salt concentrations declined due to leaching by irrigation water in four artificial reclamation treatments. This was supported by our observation that soil EC increased with soil depth in each treatment (
Figure 7). The four artificial reclamation treatments not only significantly reduced soil pH and soil salinity, but also significantly increased soil nutrient content. The increase in soil organic matter, available N, and available P varied among the four artificial reclamation treatments. Soil organic matter content under TW was nearly twice that under CK. One possible explanation for this is the treatment used in natural resources (e.g., light, water, and nutrients) more efficiently than the other artificial reclamation treatments. It should also be noted that the amount of irrigation water was highest in the TW treatment. This could have contributed to the accumulation of soil organic matter.
Vegetation cover varied among four artificial reclamation treatments, leading to the differences in both mineralization of soil organic matter and soil nutrient content. These differences had a significant impact on soil microbial populations, especially in the TW treatment. The TW treatment also increased nutrient exchange between plant and soil. These conditions drove soil microbial populations to rise. Both TW and CA could be recommended for reclamation of abandoned, salinized farmland. The artificial reclamation treatments also increased soil microbial populations (
Figure 4), SMBC and SMBN (
Figure 5). Among all the treatments, TW had the best performance, increasing SMBC by 43.0% and SMBN by 88.6% compared with CK. The results indicate both TW and CA provided soil microorganisms with sufficient organic C and N. This improved soil physicochemical properties and increased soil microbial populations. The use of fertilizers increased the decomposition of soil organic C and reduced the ratio of soil carbon and nitrogen. A reduction in the availability of soil organic C decreased not only microbial activity but also soil microbial biomass C and N [
35]. Therefore, TW seems to be an optimal reclamation method to increase soil quality.
Soil qSMBC and SMBC responded differently to four artificial reclamation treatments (
Figure 5). Specifically, CA and TS significantly increased qSMBC. A possible explanation was that legume roots secrete large amount of sugars, amino acids, and other low-molecular organic compounds. These compounds could stimulate the growth of microorganisms in the rhizosphere [
36]. If legumes were planted on abandoned farmland, the mass fraction of soil organic C would increase significantly. High soil organic matter content would provide sufficient C, N, and energy for microbial growth. The soil microbial quotient was significantly higher in TS than in the other treatments. One explanation was that trees can pull more water from the soil and reduce the groundwater level. TS significantly reduced soil salinity [
37]. Furthermore, the soil in TS was less disturbed than in CC, CA, and TW. Land use type had less influence on qSMBC than on SMBC, which was consistent with the observations by Zeller et al. [
38], but different from the report of Saggar et al. [
39], which stated that qSMBC was more sensitive than SMBC to changes in cultivation. Overall, the results indicate that the turnover of SMBC was highest under CA. This would promote the conversion of SOC into SMBC. Studies on the variation of SMBC and qSMBC during the reclamation process of abandoned salinized farmland could either directly or indirectly reflect the improvements in soil quality under different reclamation practices.
Soil nutrient availability to plants was directly related to soil enzyme activity. Mineralization of soil nutrients increased as soil enzyme activity increased. Nutrient recycling in the soil increased as mineralization increased [
40]. Compared with CK, four artificial reclamation treatments increased the activity of urease, sucrase and phosphatase (
Figure 6). Urease and phosphatase activity were highest under TW, suggesting that this treatment would increase both N use efficiency and available soil P content. Sucrase activity increased most under either CA or TW. Overall, these results indicate that soil quality increased more rapidly under human management than under CK.
Economic profit is an important benchmark to evaluate reclamation treatments. Cotton was widely grown in this region. The economic value of wood production in both TS and TW should not be overlooked. Trees will take 12–20 years before they mature. However, when calculated on annual basis, the estimated economic returns were as follows: 92,500 Chinese Yuan (CNY) ha−1·year−1 for TS, 71,000 CNY ha−1·year−1 for TW, 13,500 CNY ha−1·year−1 for CC, and 7,500 CNY ha−1·year−1 for CA (the exchange rate of CNY against US Dollar (USD) is 6.75 and the exchange rate of CNY against Euro (EUR) is 7.47 on 2 November 2016). If both economic benefits and ecological effects are considered, then TW was the best means for reclamation. The findings of this study have demonstrated the variation of different reclamation treatments in abandoned, salinized farmland. Four artificial reclamation treatments resulted in an improvement in soil quality and ecological environment, which was conducive to the sustainable development of the farmland ecosystem.
5. Conclusions
The focus of this study is to assess the effect of soil fertility improvement and soil salinization control under different reclamation treatments. Results show that all four artificial reclamation treatments significantly increased soil nutrient content. The magnitude of soil nutrient content followed the order of TW > CC > CA > TS > CK. Soil microbial population and biomass were highest under TW and CA. The activities of soil urease, sucrase, and alkaline phosphatase were higher under the four artificial reclamation treatments than under CK. Overall, TW and CA were the preferred treatments for reclaiming abandoned farmland. In terms of the ecological benefits, TW and CA were the best treatments. If both economic effects and ecological benefits are considered, then TW was the best means for reclamation. The integration of soil ecological environment and economic benefits can provide a comprehensive assessment and a guide for decision makers to achieve sustainable land resource utilization in arid areas.
Although this study demonstrates that an optimal reclamation treatment can be selected based soil nutrient, microorganism and enzyme activity, further research is needed to focus on investigating other reclamation treatments to ensure an adequate and detailed abandoned land reclamation assessment. These include the assessment of agricultural water resource management, sustainable land use and the ecological environment.