Differences in organic carbon accumulation between reddish paddy and upland soils under long-term fertilization

Objectives Increasing soil organic carbon level is of great significance for improving farmland productivity. The accumulation characteristics of organic carbon were studied under long-term different fertilization modes in a reddish paddy soil and an upland red soil that both developed from the same parent material, which would provide scientific basis for further understanding the underlying mechanism of organic carbon sequestration and stabilization in fertilized red soils.

Methods The study included two long-term fertilization experiments, one was in paddy field and started in 1981, and the other was in adjacent upland field and started in 1986. Surface soil samples (0‒20 cm) were collected after harvest of late rice and maize in November 2017 in the three treatments of CK (no fertilizer), NPK (chemical N, P, K fertilizers) and NPKM (chemical NPK fertilizers plus organic manure) of both the experiments. The contents and storages of TOC and H₂SO₄-hydrolyzed labile and recalcitrant organic carbon in soils were measured. In addition, soil TOC sequestration potential was computed by fitting the dynamics of TOC content using Jenny model.

Results Fertilization increased the contents of all organic carbon fractions in soils, and NPKM treatment increased more than NPK did. The content increase range of all the organic carbon fractions in the upland soil was significantly higher than that in the paddy soil. Under NPK treatment, the increase ranges of labile organic carbon fraction Ⅰ (LOCF-Ⅰ), labile organic carbon fraction Ⅱ (LOCF-Ⅱ) and recalcitrant organic carbon (ROC) in upland soil were 2.7 times, 2.7 times and 5.8 times of those in paddy soil, respectively, while under NPKM treatment, the increase ranges of LOCF-Ⅰ, LOCF-Ⅱ and ROC in upland soil were 2.0 times, 1.4 times and 2.5 times of those in paddy soil, respectively. No matter fertilization or not, the TOC sequestration amount and potential in paddy soils were significantly higher than those in upland soils. NPKM treatment presented better effect for promoting TOC sequestration as compared with NPK treatment. The soil TOC sequestration amount and potential under NPKM treatment were 1.7 times and 1.4 times of those under NPK treatment in paddy field, and were 25.5 times and 5.8 times in upland field. Long-term fertilization significantly increased exogenous carbon input in both paddy and upland fields, and the increase of labile organic carbon storage contributed 64.7% of TOC storage in paddy soil and 44.6% in upland soil, respectively. The contents of LOCF-Ⅰ, LOCF-Ⅱ and ROC in the paddy soil were all significantly higher than those in the upland soil, regardless of fertilization regimes. The difference in total labile organic carbon (summing LOCF-Ⅰ and LOCF-Ⅱ) content between the paddy and the upland soil could explain 52.9%‒60.0% of the difference in TOC content.

Conclusions Manure amendments combined with chemical NPK fertilizers significantly promote organic carbon sequestration in red soils as compared with chemical NPK fertilizers application alone, which is more pronounced in upland system. Compared with paddy soil, the change of content of each organic carbon fraction in upland soil was more sensitive to fertilizations, which is more obvious under the condition of chemical NPK fertilizers application. The main contributors of TOC accumulation in paddy and upland red soil are soil labile and recalcitrant organic carbon, respectively. Though reddish paddy soil is beneficial for TOC sequestration as compared with upland red soil, the proportions of labile fractions in its TOC is relatively higher, and thus is vulnerable to loss due to improper agricultural management.