Straw return is an important farmland management practice that influences the activity of soil nitrogen. Few studies have examined the distribution of soil nitrogen and its components in wheat–rice cropping fields in subtropical China. This study assesses the influence of different years of straw return on the distribution and variation of total soil nitrogen (TN), light fraction nitrogen (LFN), heavy fraction nitrogen (HFN), particulate nitrogen (PN), and mineral-bound nitrogen (MN). We conducted a field experiment with eight years of straw retention treatments in 2017 (no straw retention, NR; 1 year of straw retention, SR1; 2 years of straw retention, SR2; 3 years of straw retention, SR3; 4 years of straw retention, SR4; 5 years of straw retention, SR5; 6 years of straw retention, SR6; 7 years of straw retention, SR7) and one more treatment in 2018 (8 years of straw retention, SR8) in a rice–wheat cropping system at Yangzhou University Experimental Station in China. The results demonstrated that as the number of years of treatment increases, the content of TN, LFN, HFN, PN, and MN at each soil layer gradually increases. Compared with NR, the highest increase in TN, LFN, HFN, PN, and MN under SR1-SR8 in the 0–20 cm soils was 38.10%, 150.73%, 35.61%, 79.97%, and 27.71%, respectively, but increases in TN, HFN, and MN content gradually slowed after six years of straw return. The contents or variation of TN were extremely significantly correlated (p < 0.01) with that of LFN, HFN, PN, and MN, while LFN had the highest variation. In general, straw return could improve the quality of the 0–20 cm nitrogen pool. LFN was the best indicator of changes to the soil nitrogen pool affected by years of straw return.
Crop straw is an important natural resource in China because it is rich in nutrients. When returned to fields after harvests, the straw can improve soil quality and the next crop's yield. Evaluating the economic values of the main ecological services of a farmland ecosystem while implementing the straw return technique can be a more systematic and comprehensive approach to better understand the contribution of straw return to the development of ecological agriculture. Based on the data of a field experiment established in 2010 with varying numbers of years of straw return, four ecological services, i.e., agricultural product and industrial raw materials, atmospheric regulation and purification, soil nutrient accumulation, and water conservation, were selected to estimate a net ecosystem service value (ESV) of a wheat field's ecosystem services. Agro-ecosystem service appraisal theories were applied to estimate the economic value of each service. Results showed that straw returning improved the total ESV in the wheat system. Compared to the no straw return treatment, 1 year, 3 years, 5 years or 7 years of straw returning altered the economic value of the agricultural product and industrial raw materials (EVAIM) by -5.93% to 7.84% and improved atmospheric regulation (EVAR) by 13.66%-30.80%, soil nutrient accumulation (EVSNA) by 59.87%-233.31% and water conservation (EVWC) by 2.60%-13.26%. The total ESV of wheat plots with 1-7 years of straw returning was 3.67%-27.41% higher than that with no straw return, and the total ESV increased with the increase in years of straw return. The proportion of EVAIM out of the total ESV in this wheat field system was highest (accounted for 47.09%-55.64%), followed by EVAR and EVWC. The value of EVSNA was the lowest. However, the proportion of EVSNA was higher than that of water conservation after the fifth year of straw return. In general, the adoption of continuous straw returning in a wheat field ecosystem is ecologically valuable. The results can inform the development and implementation of ecological compensation policies involving straw return.
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