Effects Of Leguminous Green Manure On Soil Water And Nitrogen Supply And Its Mechanism In The Loess Plateau

As a typical rain-fed agricultural region, the sustainable agricultural development and crop production of dryland farming in the Loess Plateau of China is gradually facing the severe challenge of water deficiency and poor soil fertility. Growing legumes as the green manure(GM) crop during the summer fallow period is a viable method to improve soil fertility, subsequent crop growth and productivity, and increase the utilization efficiency of natural resources in time and space. A long-term field experiment(started in 2008) was conducted to investigate the effects of leguminous GM crops {Huai bean(Glycine ussuriensis Regel et Maack.), soybean [G. max(L.) Merr.], and mung bean(Phaseolus radiatus L.)} and the N fertilizer rates(0, 108, 135, and 162 kg N/ha) on soil water and nitrogen(N) supply, and its mechanism in the Loess Plateau. The results of this study will be used to develop a GM-based cropping system, providing a theoretical basis and guiding field management strategies for local and other similar dryland regions. The main results were as followed:(1) The 4-yr results of this long-term field experiment with different leguminous GM approaches indicate that planting legumes as the GM crop stimulated the subsequent winter wheat(Triticum aestivum L.) growth at the early growth stage(the number of tillers per plant and stems per hectare), and improved the productivity(yield, biomass and number of spikes per hectare) and nutrient uptake compared to bare fallow. However, high expenditures for field management and variable weather patterns led to few direct economic benefits of the GM approach. Besides, application of N fertilizer significantly increased wheat growth at the early growth stage, productivity, nutrient uptake, and gross and net returns; however, there was no significant difference among the three N fertilizer rates.(2) According to the 6-yr results of this long-term field experiment combined with the 57-yr annual precipitation in the Weibei area, we investigated the temporal characteristics of precipitation to better understanding the patterns of the wheat yield, water use efficiency(WUE), and soil water balance(SWB) under the GM approach in different precipitation years. Results confirmed that planting legumes as the GM crop consumes soil water in the fallow season can bring variable effects to the subsequent wheat. The effect is positive when the annual precipitation is adequate, so that there is no significant reduction in the soil water supplied to wheat. If this is not the case, the effect is negative. On average, the GM approach increased wheat yield and WUE by 13% and 28%, respectively, and efficiently enhanced the SWB(0-200 cm) compared with the fallow management.(3) A 374-days field decomposition trial was conducted to investigate the mechanism of GM N via decomposition, transformation, utilization, and relative contribution to grain production in the dryland traditional winter wheat cropping system. Results showed that GM legumes accumulated 53~76 kg N/ha per year. After decomposing for one year, more than 32 kg N/ha was released from GM legumes. The amount of N released via GM decomposition that was subsequently utilized by wheat was 7~27 kg N/ha. By managing for balanced N inputs and outputs, the mean total N input in the GM-based systems was 164 kg N·ha-1·yr-1, and the mean harvested N export was 114 kg N·ha-1·yr-1, leaving a surplus of 50 kg N·ha-1·yr-1. Incorporation of GM legumes effectively replaced 13~48%(on average 31%) of the applied mineral N fertilizer. Additionally, the GM approach during the fallow period reduced the risk of nitrate-N leaching to depths of 100 to 200 cm by 19.6 kg N/ha.(4) The 5-yr results of this field experiment was used to evaluate the effect of GM legumes on soil organic carbon(SOC) and total N(TN) stocks(0-20 cm depth). Besides, we also estimated the dynamics of SOC stock for the GM systems under dryland condition by using the one-compartment model. Analysis of the GM legume biomass, carbon(C) and N accumulations suggested that there were significant power relationships with the plant growth. The percentages of remaining GM C and N in the soil after a one year field incorporation averaged to 26% and 31% for the above-ground residues, and averaged to 33% and 52% for the below-ground residues, respectively. Planting the summer legumes as the GM crop for continuous 5-yr significantly improved SOC, liable organic carbon(LOC), and TN concentrations and built up the corresponding stocks at topsoil compared with the original stocks of native cropland under the dryland condition. There appeared a close linear relationship between annual C input by crops and SOC stocks in the 5th year. The minimal annual C input required to maintain the original SOC stock(19.04 Mg C/ha) was equivalent to 0.68 Mg C·ha-1·yr-1. Based on the concepts of the one-compartment model, we estimated the annual decomposition rate of SOC as 1% and the C added by crops remained in the SOC as 23% in each year. Besides, the mean turnover time(Tturnover) of C in SOM at equilibrium was estimated as 22 years, suggesting that this loess soil was not C saturated and still had potential for C sequestration.Overall, the inclusion of the GM crops in the traditional winter wheat- summer fallow cropping system in the Loess Plateau could not only stimulate the subsequent winter wheat growth at the early growth stage and nutrient uptake, and improve the productivity and WUE, but also efficiently replace the mineral N fertilizer applied to wheat. Besides, growing the legumes as the GM crops had the potential to reduce the risk of nitrate-N leaching to the deeper depth and obviously enhanced the topsoil fertility under the dryland condition. Consequently, it is feasible and profound to develop the GM-based cropping system in the Loess Plateau, which will be helpful to the establishement and development of the efficient and environment friendly modern agriculture in China.