Straw amendment induced contrasting net carbon balance in subtropical paddy and adjacent upland soils

Soil organic carbon (SOC) sequestration in agricultural fields is of vital importance to mitigate climate change. Straw amendment is a promising practice for improving SOC sequestration through direct input of organic materials. Subtropical paddy soils have higher SOC levels than upland soils, but whether or not the decomposition of straw C and the net soil C balance contribute to higher SOC levels in paddy soils remains unknown. Here, two typical subtropical paddy soils derived from highly weathered granite and quaternary red clay, as well as adjacent upland soils, were incubated at 25 °C and 45 % water holding capacity for 180 days. Two treatments of soil alone (Control) and soil with straw amendment (M) were conducted. About 18–21 % of straw C evolved as CO₂ in paddy soils, which was significantly lower than in upland soils (23–28 %). Additionally, straw amendment induced weaker priming effect (PE) in paddy (5–9 %) than in upland (17–37 %) soils. However, more straw C was incorporated into SOC in paddy (29–33 %) than in upland (26–29 %) soils. Consequently, straw C led to a positive net C balance in paddy soils (from 128 to 372 mg C kg⁻¹), but a negative net C balance in upland soils (from −313 to −111 mg C kg⁻¹). The straw C incorporated into SOC was distributed mainly in the humus C, Fe/Al–SOC, particulate organic C fractions, and macro-aggregates (2000–250 μm),with higher proportions of 14–15 %, 17–24 %, 15–21 %, and 10–13 % in paddy than those of 11–13 %, 13–18 %, 9–17 %, and 3–7 % in upland soils, respectively. Straw amendment to subtropical paddy soils may offset the SOC losses and result in net SOC sequestration via the reduction of substrate mineralization, weaker PE, and stronger physicochemical protection of SOC than in upland soils. On the contrary, straw amendment to subtropical upland soils is not conducive to SOC sequestration. This study highlights the contrasting roles of straw decomposition in subtropical paddy and upland soils. It suggests that straw amendment is a reasonable management practice to improve net SOC accumulation in paddy soils, but this practice may not be suitable in upland soils.