Nitrogen dynamics and mineralization in degraded agricultural soil mulched with fresh grass

Understanding mulching influences on nitrogen (N) availability is important for developing N management strategies in plantations at the upland sites of the southwestern China. Dynamics of biomass loss and nutrient release of mulching material, N availability in the soil and N mineralization in situ were evaluated for the treatments with different mulch quantity in degraded agricultural soil. The time taken for 95% decomposition of the initial biomass of Cogon grass (Imperata cylindrical L. Beauv. var. major) was 17 months with a half-life (t _1/2) of about 4.8 months. During the first 4 months about 55.2% of N was released, and after 1-year decomposition about 71.6% of N was released from the mulch material. The fresh grass mulch increased the available N in the soil as they decomposed. Compared to no mulch treatment, mulch treatments with 2.5, 5.0 and 7.5 kg m⁻² mulching grass increased available N by about 13.1, 40.8 and 56.4% in the top soil (0–5 cm), and about 23.6, 78.0 and 139.3% in the middle layer (5–20 cm), respectively. The mean annual net N mineralization in the mulched plots had 9.0–40.9% higher cumulative rate than that in no-mulch plots, and the majority of the accumulated N in the incubated soils existed as NO₃–N. There was a positive relationship between the rate of N mineralization and the available N in both the top soil and the middle layer. Mulch improves soil nutrients and this improvement increased with increasing mulching quantity. The increment of net N mineralization was approximately 69, 161 and 322 kg N ha⁻¹ year⁻¹ in the soil of 0–20 cm depth for the 2.5, 5.0 and 7.5 kg m⁻² grass mulch treatments, respectively. The results from this study will provide a basis to optimize mulching techniques for poplar plantations in degraded agricultural soils of southwestern China.