The Dynamic Changes Of Soil Organic Carbon And Crop Yield On Farmland In Hilly And Gully Areas Of The Loess Plateau

As a typical rainfed agricultural area in China, the Loess Plateau region has poor soil and severe water shortage, seriously hindering the stability of agricultural production and its sustainable development. To study how to increase soil carbon sequestration (soil fertility) and yield-increasing, is important and necessary not only for the sustainable development of agricultural in this region as well as our country, but also for the global atmospheric CO2 mitigation. Previous researchers through conventional field observation methods had proved that to increase fertilizer input was the key measure to enhance soil carbon sequestration and crop yield-increasing. Therefore, to study the effects of different fertilization on soil carbon cycling rules, SOC content change trends and crop yield-increasing trends, could help us reveal the characteristics of soil carbon storage whether as a sink or a source, contributing to soil fertility and food security. In this study, DNDC model was used to simulate and predict both the farmland SOC content and crop yield dynamic changes under three types of fertilization treatments, single fertilizer application, single manure amendment and combined application of fertilizer and manure, as well as a no fertilizer treatment. Soil carbon cycling rules, SOC content changes, yield-increasing trends and so on were investigated by the simulated results. The main obtained results are as follows:(1) The credibility of DNDC model's simulation results could only be determined by comparing with the measured data. In this study, the long-term observed data collected from three long-term fertilization experiments, supported by Ansai Soil and Water Conservation Experimental Station, were compared with the simulated dynamic changes of SOC content and crop yield, in order to further verify the applicability of DNDC model in hilly and gully areas of the Loess Plateau. The validation results showed that the comparisons between observed and the simulated SOC content and crop yield were all significance at 0.01 level, suggesting the DNDC model can be used to simulate and predict both the farmland SOC content and crop yield dynamics.(2) Among the environmental factors, initial SOC content was the most sensitive factor for regulating SOC sequestration rate as a"carbon source"; among the farm management factors, crop residue incorporation and manure amendment are the most sensitive factors affecting the SOC dynamics as a"carbon sink". Results from the sensitivity tests have a dual implication, which indicate that SOC dynamics is sensitive to some input factors (e.g., initial SOC content, crop residue incorporation, manure amendment) more than others. It also revealed there are potentials for enhancing SOC sequestration through improved farming management practices.(3) On the three types of farmland, fertilizition could significantly increase crop residues and root exudates amount, bringing more exogenous C into soil; it also could significantly enhance soil heterotrophic respiration, increasing endogenous C consumption. That suggesting it could both increase soil carbon pool's capability as "carbon sink" and "carbon source", and single manure and combined applications had more significant effects. On the slope land and plain field, no fertilizer made their soil fertility declined due to soil nutrients stress, even though soil fertility on terrace was improved, due to the introduction of a leguminous crop, soybean. Single nitrogen fertilizer application on the slope land and plain field, could ensure crop production, but the surface soil's fertility improvement was not obvious, or even be reduced slightly. On the three types of farmland, single manure or combined application both could increase SOC content significantly, and the fertilizing effect of combined nitrogen fertilizer with manure application is better.(4) On slope land and plain field, no fertilizer made their crop yields declined continually due to a continuous deterioration in soil fertility, on terrace, the crop yield was promoted due to the introduction of soybeans, a leguminous crop. Single nitrogen fertilizer application, on slope land and plain field, could ensure crop production basicly, but had an serious effect on soil fertility. Therefore, on the three types of cultivated land, combined application of nitrogen fertilizer and manure should be advised as a better yield-increasing measure in this region.(5) On the barren farmland, the necessary measures would not only enhance its soil fertility, but also play a very significant role in promoting crop yields. However, when the soil fertility beyond a threshold value that might exist, other factor, such as annual precipitation, mean annual temperature, crops'own productivity differences and so on, would replace soil fertility, becoming a new limiting factor for crop production. Standing on the sustainable development of agriculture, in this study, the combined application on slope land, terrace field and plain field should be a better choice, which would achieve the win-win goal of soil carbon sequestration and food production security.