Long-term changes in topsoil chemical properties under centuries of cultivation after reclamation of coastal wetlands in the Yangtze Estuary, China

Dynamics of reclaimed coastal wetland soils under cultivation has not been well understood, especially at temporal scales longer than a century. In this study, we analyzed major chemical properties of plow-layer soils extensively sampled under two cropping systems (paddy rice vs. upland cropping) along a 500-year soil chronosequence created by intermittent reclamation of coastal salt marshes. The results suggested a rapid desalinization of soil immediately after reclamation. During 500 years of cultivation, the decalcification process lowered soil pH from >8 to nearly neutral. Soil organic carbon (SOC) contents markedly declined in the initial 16 years, but then rapidly recovered within 30 years and thereafter slowly accumulated with cultivation duration. Meanwhile, the recalcitrance of SOC increased. Soil nutrient status was enhanced after centuries of cultivation as indicated by the improved total nitrogen (TN) and phosphorous (TP). Amorphous Fe oxyhydrates progressively decreased, but the crystallinity of Fe oxyhydrates increased with cultivation time. Cropping system greatly affected plow-layer soil properties, with paddy soils having higher SOC, MBC, NH4OAc-extractable Ca2+, but lower TP, NH4OAc-extractable K+, potentially mineralizable nitrogen and Fe crystallinity than upland soils. Most soil properties revealed clear temporal patterns with more remarkable changes occurring in the first several decades after reclamation than in the following centuries. In conclusion, there was a transition phase of the soil system within the first several decades after reclamation of coastal wetlands, possibly harmful to agricultural production, but centuries of cultivation seemed to have significantly improved overall soil fertility.