Seasonal and long-term changes in soil physical properties and organic carbon fractions as affected by manure application rates in the Mollisol region of Northeast China

Organic amendments may increase crop yields by improving soil nutrient availability and soil structures by enhancing soil organic matter (SOM). Yet how soil structures are improved by increasing organic application rates during crop growth periods and affect crop yields are not well understood. By using a 14-yr field experiment, the objectives of this study were 1) to examine long-term effects of amendment rates of organic manure on crop yields, soil organic carbon (SOC) contents in aggregates and bulk soil, and soil structural and hydraulic properties, and 2) to investigate the effects of SOC changes on the seasonal dynamics of soil aggregates, soil pore structures and hydraulic conductivity. The organic manure was incorporated into soil at the rates of 0 (OM0), 7.5 (OM7.5), 15 (OM15) and 22.5 (OM22.5) Mg ha−1 in combination with balanced mineral N, P and K fertilizers. The manure application rates had negligible effects on soil N and K availability, but increased crop yields. The SOC contents, aggregate stabilities, and hydraulic conductivities changed with time in all the treatments during the soybean growth period in 2014, but in different manners as affected by the manure application rates. With the increase of manure application rates, SOC storages increased in all physical fractions and in the > 0.25-mm aggregates. However, the saturated hydraulic conductivity and effective porosity of > 1-mm macropores increased profoundly only in the treatments of OM15 and OM22.5 after June. The correlation analyses suggested that all the SOC pools might contribute to the stabilization of macroaggregates and that the transformation of organic manure into SOC in macroaggregates might influence the formation of the stable, > 1-mm macropores. These findings indicated that sufficient organic materials (equivalent to total annual crop straw biomass) should be incorporated into soil to maintain the resilience of soil structures in addition to high water retention and drainage capacities during the crop growth period in the high-altitude Mollisol region of China.