Effects of Rhizodeposition on Straw Carbon and Nitrogen Sequestration in Soil Profile Under Different Fertilization Conditions

Straw returning is an important measure for protecting black soil in Northeast China. Straw and rhizodeposition coexisted in the practical agricultural production, while the sequestration characteristics of straw carbon (C) and nitrogen (N) in soils remained not very clear under this condition. The purpose of this study was to compare the differences in the content of soil organic C (SOC) derived from straw C (¹³C-SOC) and that of soil total N (TN) derived from straw N (¹⁵N-TN) among different soil layers, and to investigate the effects of rhizodeposition on straw C and N sequestration in soil profile under different fertilization conditions, so as to provide a basis for the protection and utilization of black soil in Northeast China.

Based on the long-term experimental station of Shenyang Agricultural University, the treatments of adding ¹³C¹⁵N double-labeled straw (S) and its combination with rhizodeposition (hereafter referred to as “rhizodeposition”) (SR) were designed under different fertilization plots (no fertilization, CK; single application of chemical fertilizer, NP), including four treatments: CK+S, CK+SR, NP+S, NP+SR. The contents of SOC and TN, and values of δ¹³C and δ¹⁵N at different soil layers were measured after the 50 and 150 days of in-situ experiment.

At the early stage of straw decomposition (the 50^th day), fertilization, rhizodeposition and their interactions significantly affected (P<0.05) the contents of ¹³C-SOC and ¹⁵N-TN in the topsoil (0-20 cm). On the 50^th day, compared with the CK+S treatment, the CK+SR treatment increased the contents of ¹³C-SOC and ¹⁵N-TN in the topsoil by 18.6% and 21.7% (P<0.05), respectively. The contribution percentage of ¹³C-SOC to SOC in the topsoil was, on average, 10.5% and 12.0% in the S (CK+S and NP+S) and SR (CK+SR and NP+SR) treatments under different fertilization, respectively. The contribution percentage of ¹⁵N-TN to TN in the topsoil was, on average, higher 27.6% (P<0.05) in the two treatments under CK (CK+S and CK+SR) than that in the corresponding treatments under NP (NP+S and NP+SR). On the 50^th day, the contribution percentage of ¹³C-SOC to SOC and that of ¹⁵N-TN to TN at deep soil (20-50 cm) were 1.0%-2.2% and 0.5%-0.9%, respectively. At the later stage of straw decomposition (the 150^th day), rhizodeposition and fertilization significantly affected (P<0.05) the contents of ¹³C-SOC and ¹⁵N-TN in the topsoil, respectively. On the 150^th day, compared with the treatment of rhizodeposition, the treatment of straw addition increased the ¹³C-SOC content in the topsoil, on average, by 12.6% (P<0.05). The ¹⁵N-TN content in the topsoil was, on average, higher 22.0% (P<0.05) in the two treatments under CK than that in the corresponding treatments under NP. The contribution percentage of ¹⁵N-TN to TN in the topsoil in CK and NP treatments was 5.5% and 4.0%, respectively. On the 150^th day, the contribution percentage of ¹³C-SOC to SOC and that of ¹⁵N-TN to TN at deep soil were 0.8%-3.2% and 0.7%-1.8%, respectively.

Rhizodeposition had a negative feedback effect on the sequestration of straw C in topsoil during the later stage of straw decomposition. Straw derived C and N were constantly migrated and then accumulated from topsoil to deep soil, and their influences on the stabilities of soil organic C and N pools should be paid more attention.