Objectives The tillage layer become thin in the farmland of wheat-maize rotation system area, simply increase of tillage depth tends to deteriorate soil fertility. We studied the effects of deep tillage coupled with straw returning in the farmland through four years of continuous experiments.
Methods The experiment was carried out in Suixi County in the southern part of the North China Plain in 2012–2016. The tested soil was lime concretion black soil. Before the start of the experiment, 5 cm of soil on the surface soil in the test area was artificially peeled off to simulate the soil with topsoil thinning. The experiment consisted of 4 treatments: rotary tillage (RT), deep tillage (DT), rotary tillage with straw returning (RTS), deep tillage with straw returning (DTS). At maturity, wheat and maize yields were investigated in field. After the fourth harvest, soil samples at 0–10 cm and 10–20 cm deep were collected, the soil organic carbon components, soil nutrient content and the weight of size > 0.25 mm soil aggregates were determined.
Results Compared with the RT treatment, the DT did not increase maize and wheat yield significantly, but significantly reduced the contents of total soil organic C, organic C fractions and available K, the ratio of humic acid to fulvic acid in 10–20 cm soil layer, and the proportion of water-stable aggregates size > 0.25 mm in each soil layer. Straw returning coupled with deep tillage or rotary tillage had a good effect on increasing crop yield. The average yield increase of DTS in maize and wheat in the 4 seasons were 7.72% and 8.06%, and those of RTS were 7.55% and 7.05%, respectively. Both of DTS and RTS significantly increased the ratio of humic acid to fulvic acid in the soil. In the 0–10 cm soil layer, the contents of total and each organic C fraction, and the soil available K were higher in RTS, while in 10–20 cm soil layer, the soil humic acid, total N and available P content were higher in DTS. The proportion of size > 0.25 mm water-stable aggregates was highest in DTS in the 0–10 cm soil layer, which was increased by 23.09%, and that was highest in RTS in the 10–20 cm soil layer, which was increased by 6.32%, compared with those in RT.
Conclusions In soil with thinning tillage layer, simply deep plough cannot increase crop yield, but against the increase of soil organic C and nutrient content, meanwhile destroy the soil aggregate structure. The straw returning coupled with tillage can significantly increase crop yield, and with rotary tillage can only effectively fertilize 0–10 cm soil, while with deep tillage can increases the topsoil thickness and improves the nutrient status of the 10–20 cm soil layer, and offset the adverse effect of simply deep tillage in soil structure stability in the 10–20 cm soil layer.
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