[Effects of the construction of fertile and cultivated upland soil layer on soil fertility and maize yield in black soil region in Northeast China]

Organic amendment return could enhance soil fertility, improve soil structure, and increase crop yield. However, how construction of soil layers can affect soil fertility and crop yield are not fully understood. We examined the effects of constructions of fertile and cultivated soil layer on soil fertility and maize yield in the upland black soil region in Northeast China, to provide theoretical guidance in increasing soil fertility and sustainable development of agriculture. Based on the combination of field plot experiments and demonstration regions, nine study sites with different ecological characteristics were selected from Heilongjiang, Jilin and Liaoning provinces from northeast China, covering dark brown, black, meadow, chernozem, albic, brown and cinnamon soils. There were three treatments in each study site, including maize straw return within 0-35 cm soil layer (CF_Ⅰ), the combination of maize straw and organic manure return within 0-35 cm soil layer (CF_Ⅱ) and conventional agricultural practice without organic amendmentas control (CK). The rate of straw return in CF_Ⅰ and CF_Ⅱ treatments were 10000 kg·hm^-2, and full straw for demonstration regions. The rate of organic manure in CF_Ⅱ treatment was 30000 kg·hm^-2. Considerable difference in soil fertility were recorded among the nine study sites with the trend of tillage layer > sub-tillage layer, especially for dark brown soil and albic soil. Soil fertility of tillage layer and sub-tillage layer was relatively low both for brown soil and cinnamon soil. The heavy clay and plow pan were pivotal limiting factors of soil fertility for the black soil and the meadow soil. Compared with CK, the concentrations of soil organic matter (SOM), available nitrogen (AN), available phosphorous (AP), and available potassium (AK) in tillage layers was increased on average by 1.85 g·kg^-1, 20.16 mg·kg^-1, 1.56 mg·kg^-1 and 17.2 mg·kg^-1 in the CF_Ⅰ and CF_Ⅱ treatments in five study sites with more than two years of treatments. The contents of SOM, AN, AP and AK in sub-tillage layer increased by 2.09 g·kg^-1, 12.06 mg·kg^-1, 2.18 mg·kg^-1 and 3.84 mg·kg^-1, compared with tillage layer. CF_Ⅰ treatment significantly enhanced the contents of SOM and AP in both tested soil layers, while CF_Ⅱ treatment significantly enhanced all fertility indices in both tested soil layers. This indicated that the increase of organic amendment return is an effective way to improve soil fertility. Maize yield fluctuated under the combined effect of climatic conditions and soil types. The significant differences in maize yield under CK, CF_Ⅰ and CF_Ⅱ treatments were observed with a trend of CF_Ⅱ > CF_Ⅰ > CK. This result indicated that the construction of fertile and cultivated soil layer could significantly increase maize yield independent of soil types. The construction of fertile and cultivated soil layer based on maize straw return or maize straw and organic manure combined return within 0-35 cm soil layer, could simultaneously increase soil fertility in both tillage and sub-tillage layer, as well as maize yield. We suggested that the selection of approaches of the constructions of fertile and cultivated soil layer should consider soil types and the sources of organic amendments. It should also give priority to soil layers rich in organic manure source to construct fertile and cultivated soil layers.