Effects of long-term no-tillage and maize straw mulching on gross nitrogen transformations in Mollisols of Northeast China

Conservation tillage practices are widely proposed as an effective management approach to guarantee soil productivity and sustainability by manipulating nutrient status, especially nitrogen (N) cycling. However, little is known about the interactive response of individual gross N transformation processes and their regulation of N availability in soil. Therefore, a 15N tracing study was carried out to investigate the effect of 12 years of no-tillage with different amounts of maize straw mulching on gross N transformation rates in Mollisols of Northeast China. Four treatments were arranged, including conventional ridge tillage (RT) and no-tillage treatments with 0 % (NT0), 33 % (NT33, 2500 kg ha-1 y-1) and 100 % (NT100, 7500 kg ha-1 y-1) harvested maize straw mulching. The results explicitly indicated that field management practices intensively controlled the dynamic N transformation between inorganic and organic forms and thus manipulated the retention and supply of soil N pool. Specifically, relative to the conventional ridge tillage, no-tillage reduced the rate of gross organic N mineralization (MNorg, from 2.47 to 1.56 mg N kg-1 d-1) but increased NH4+-N immobilization (INH4, from 3.47 to 4.83 mg N kg- 1 d-1) due to reduced soil disturbances. Increased maize residue input in no-till systems stimulated both MNorg and INH4, denoting an improved supply of available N with the enhanced retention of soil organic N in conservation tillage systems. In addition, no-tillage with maize straw mulching synchronically stimulated NH4+-N adsorption/fixation (ANH4, from 2.38 to 4.11 mg N kg- 1 d- 1) and subsequent release (RNH4ads, from 1.80 to 3.64 mg N kg- 1 d-1) processes, and such accelerated abiotic N turnover could react as an effective buffer for the supply and retention of available NH4+-N. Moreover, no-tillage with maize straw mulching considerably reduced autotrophic nitrification (from 6.39 to 3.70 mg N kg- 1 d-1) while simultaneously promoted dissimilatory NO3- -N reduction to NH4+-N (DNRA, from 0.35 to 0.85 mg N kg- 1 d-1), thus improving soil N retention capacity and reducing N loss risk. Taken together, considering the internal transformation of different soil N components, effective N cycling was gained in conservational cultivation by enhancing the retention and availability of soil N.

Automated summary

Conservation tillage enhances the retention and availability of soil nitrogen by manipulating nitrogen cycling. The study found that no-tillage reduces organic nitrogen mineralization rate but increases ammonium immobilization rate, and increased maize residue input stimulates the supply of soil organic nitrogen. Additionally, no-tillage promotes the adsorption/fixation and subsequent release of ammonium, reduces nitrification, and lowers the risk of nitrogen loss.