Effects of long-term straw application and groundwater management on acidification of paddy soils in subtropical China: Insight from a 35-year field experiment

As an important renewable agricultural by-product, straw application has been recommended worldwide as an effective and economical agricultural management practice. However, there remain potential risks associated with straw application, especially straw-induced soil acidification in croplands. Until now, the effect of straw application on soil acidification is inconclusive, and there is still a lack of knowledge concerning the combined impacts of groundwater management and straw application on soil acidification. A 35-year field experiment was conducted in subtropical China to assess the effects of long-term straw application and groundwater management on the acidification of paddy soils. The experiment was set up with a two-factor design of groundwater table depth (shallow groundwater table (SGT): 20 cm depth; deep groundwater table (DGT): 80 cm depth) and straw application rate (CK: chemical fertilizers with no straw application; OM1: straw substituting with one-third of N fertilizers, conventional application; OM2: straw substituting with two-thirds of N fertilizers.). After 35 years of treatment, soil pH in the 0–10 and 10–20 cm layers decreased by 0.81–1.30 units under all six treatments, while soil pH in the 20–40 cm layer decreased only under the SGT treatment. In the upper two layers, due to the high initial soil pH (6.16) and low base cations in the applied straw, straw application promoted the dissociation of straw-derived weak acids, the leaching of dissolved carbon, and the nitrification of mineralized straw-derived organic N. As a result, it increased H+ production and leaching of base cations and further led to the decrease of soil pH. In the subsoil layer, deep groundwater table management inhibited the dissociation of H2CO3 and N transformation by influencing redox conditions in the soil and compensated for the negative effects of straw application on soil acidification. This study demonstrated that straw application accelerated the acidification of paddy soils in subtropical regions and the importance of groundwater management to mitigate this adverse effect. It is proposed that rational groundwater management with alternating wetting and drying cycles could be adopted to effectively mitigate the straw-induced acidification of paddy soils. Nonetheless, considering the differences in soil properties and environmental conditions, further studies are still required to explore the potential mitigation effect of the proposed groundwater management in other agroecosystems.