Soil Water And Nutrient Supply Affected By Straw Retention And Planting Green Manure During Summer Fallow And Its Mechanism In Dryland

The excessive application of synthetic nitrogen(N) fertilizer has caused not only low N use efficiency, but also a host of environmental problems. So far, agriculture faces great challenges to ensure global food production and security while reducing the negative effects of nitrogen fertilizer on the environment. Therefore, it has led to an increasing interest of pursuing more sustainable pathways for increasing crop production in an environmentally–friendly manner. In addition, water shortage and low water use efficiency are two key factors limiting agriculture production in many areas of the world. Many researchers have suggested that developing rain-fed agriculture and increasing crop productivity on dryland is an effective approach for solving water shortage and ensures global food production security.The Loess Plateau regions in northwest of China are typical rain-fed agriculture areas. Water scarcity and poor soil fertility are two major factors limiting northwest dryland farming. The reasonable summer fallow managements of soil are crucial for improving soil fertility and increasing crop yield in dryland areas. Straw retention can retain water, regulate nutrient supply, increase soil fertility, improve soil quality, and increase crop yields. Green manure is the soil-improving crop, which not only increases soil fertility, but also improves the soil quality. Therefore, using different cultivation methods in the dryland farming, especially straw retention and green manure planting are the efficient options to resolve the drought and poor soil fertility problems in the northwest area of China. With bare fallow in summer as control, field experiments were carried out to determine the effects of straw retention and planting green manure during summer fallow on soil water and nutrient supply and its mechanism on dryland of the Loess Plateau, China. The main objective of this study was to evaluate the effects of different cultivation methods, and provide theoretical supports and practical experience for improving soil fertility and crop productivity on dryland of the Loess Plateau. The main results were as follows:(1) With bare fallow in summer as a control, a three-year location-fixed field experiment was conducted in the Loess Plateau to investigate the effects of wheat straw retention, green manure planting, and their combination on winter wheat yield, soil water retention during summer fallow, and crop water use and nitrogen uptake. Results showed that straw retention increased soil water retention during summer fallow by 20 mm on average compared with the control over three experimental years but reduced the grain yield by 8% in the third year and the grain nitrogen content by 6–15% in all three years. In contrast, green manure planting markedly reduced soil water retention by 16 and 33 mm in the first and third year, respectively, but increased water use efficiency by 16% in the third year and nitrate nitrogen accumulation in 0–100 cm soil at winter wheat sowing. Their combination did not significantly affect the soil water retention or the soil nitrate nitrogen content in any of the three years, but did increase water use efficiency by 11% in the third year and grain yield by 2.6% in the second year. In conclusion, the combination of straw retention and green manure planting mitigated the negative effects of the individual measures, providing a feasible method for summer fallow management in the semiarid Loess Plateau in China and other similar regions.(2) A two–year field micro–plot experiment was conducted to investigate green manure, straw and fertilizer nitrogen–15(15N) taken up by winter wheat, the residue in the soil and the loss under four different summer fallow managements in the Loess Plateau. Results showed that green manure N taken up by wheat was 15.4 kg ha-1(22.4%), residual in soil was 35.4 kg ha-1(51.7%), and that lost was 17.7 kg ha-1(25.9%) during the two experimental years. Addition of wheat straw significantly decreased the green manure N residual in the soil but increased the accumulative N loss. In addition, straw N taken up by wheat was 2.0 kg ha-1(8.3%), residual in soil was 7.5 kg ha-1(31.0%), and that lost was 14.6 kg ha-1(60.7%). Straw N taken up by wheat was significantly decreased by the addition of green manure, that residual in the soil was increased while the lost was reduced. Furthermore, 44.7 kg ha-1(32.4%) inorganic fertilizer N was taken up by winter wheat, 44.6 kg ha-1(32.3%) was resided in the soil, while that lost was 48.6 kg ha-1(35.2%). Addition of crop residues significantly increased the fertilizer N taken up by wheat in the second year but decreased that resided in the soil. Overall, our results suggested that the application of inorganic N fertilizer in combination with appropriate crop residues is an effective approach to enhance fertilizer N efficiency in the long–term, and thus offers the potential for the improvement of soil quality, crop yield and decrease fertilizer N loss in wheat–summer fallow agricultural systems, especially on dryland.(3) Combing 15 N tracer techniques, an incubation experiment was conducted to investigate the mineralization of N in the shoots and roots from leguminous and non-leguminous green manure and also analyze the relationship between characteristics of different crop residues and theirs N mineralization. Results showed that leguminous green manures exhibited net N mineralization during the whole incubation period, and decayed fast in the first month. In contrast, non-leguminous green manures showed N immobilization in the first month. At the end of incubation(100 d), net N mineralization rates in leguminous green manures were significantly higher than that of non-leguminous green manure. The average 15 N net mineralization rates of leguminous green manures were 33.1%, while only 19.7% for non-leguminous green manures. Overall, N mineralization rates of aboveground parts were higher than these of the roots. Compared with the roots, 15 N net mineralization rates from aboveground parts of green manures increased by 1.4–15.3%. Net N mineralization rates of crop residues were negatively and linearly correlated with their C/N ratios, lignin, cellulose and hemicelluloses. Furthermore, C/N ratios could be a better indicator for their N mineralization. Therefore, the retention of crop residues with lower C/N ratios should synchronize nutrients release with crop demands, and applying crop residues with higher C/N ratios should be in combination with appropriate N fertilizer to avoid N competition between the decay of residues and crop growth.(4) With bare fallow as a control, a three-year location-fixed field experiment was conducted in the Loess Plateau to investigate the effects of wheat straw retention, green manure planting, and their combination on phosphorus(P) and potassium(K) uptake by winter wheat, soil available P and K. Results showed that planting green manure and their combination treatments during summer fallow period decreased soil available P in the 0–40 cm layers compared with the control treatment, but planting green manure increased soil available P in the surface soil at the flowering stages of the three years and harvest of the first year but did not show any significant differences on the grain P content and aboveground P uptake. Compared with the control, straw retention increased soil available P in the surface soil at flowering stages of the three experimental years and harvest of the first year, but significantly reduced aboveground P uptake by 8.4% in the third year. Straw retention significantly reduced aboveground K uptake by 14% in the third year and did not show any significant differences on soil available K amount. Planting green manure and their combination treatments did not show any significant effects on soil available K amount and aboveground K uptake. In conclusion, planting green manure during summer fallow decreased available P in soil surface layers at sowing but increased that at flowering and harvest stages. Straw retention reduced aboveground P and K uptake in the third year. However, the combination of straw retention and green manure planting could slow down the decrease of aboveground P and K uptake, thus it provided an effective method for summer fallow soil management.