Carbon isotopic measurements from coastal zone protected forests in northern China: Soil carbon decomposition assessment and its influencing factors

The effect of thinning intensity on the soil carbon pool mediated by soil microbial communities and necromass carbon in coastal zone protected forests

Thinning is a common forest management measure that can effectively maintain the ecological service function of protected forests. However, the effect of thinning on the soil carbon (C) pool remains uncertain. In particular, we lack an understanding of the complete link between thinning and microbial communities, microbial necromass C, and consequently, soil C pools in coastal zone protected forests. In this study, three thinning intensities, i.e., a control treatment (CT, i.e., no thinning), light thinning (LT) and heavy thinning (HT), were established in three types of forests (Quercus acutissima Carruth, Pinus thunbergii Parl and mixed Quercus acutissima Carruth and Pinus thunbergii Parl, i.e., QAC, PTP and QP, respectively). Two years after the completion of thinning, we investigated the changes in the soil organic carbon (SOC) fractions, soil microbial community and soil microbial necromass C in the surface layer (0-20 cm) and thoroughly evaluated the relationship between the potential change in SOC and the microbial community. Compared with CT, there was no change in the SOC content under LT and HT, but thinning conducted in QAC increased the proportion of mineral-associated organic C (MAOC) in SOC. Moreover, both LT and HT reduced the soil carbon lability (CL) in the QAC and QP forests. Different thinning intensities changed the soil microbial community structure, and most of the variation was explained by thinning and the soil physicochemical properties. The proportion of soil bacterial and fungal necromass C to SOC increased with increasing thinning intensity. The content of soil bacterial and fungal necromass C was mainly controlled by the relative abundance of the core phylum (relative abundance>10 %). Thinning affected the soil C pool by affecting the content of soil bacterial and fungal necromass C, but their accumulation pathways was different. The results showed that thinning was beneficial to the stability of SOC. The microbial C pool, total organic C pool and even bacterial and fungal C pools should be distinguished when studying the soil C pool, which can effectively deepen our understanding of the mechanism by which soil microorganisms affect the soil C pool.

Effects of urbanization intensity on glomalin-related soil protein in Nanchang, China: Influencing factors and implications for greenspace soil improvement

Glomalin-related soil protein (GRSP) is a stable and persistent glycoprotein secreted by arbuscular mycorrhizal fungi that plays an important role in sequestering soil organic carbon (SOC) and improving soil quality. Rapid urbanization disturbs and degrades the soil quality in the greenspace. However, few studies have investigated the effects of urbanization on GRSP and its influencing factors. This study selected impervious surface area as a measure of urbanization intensity. A total of 184 soil samples were collected from the 0-20 cm soil layer in the greenspace of Nanchang, China (505 km²). The GRSP content, soil properties, urban forest characteristics, and land-use configuration were determined. The total GRSP (TG) and easily extractable GRSP (EEG) contents were 2.38 and 0.57 mg g^-1, respectively. TG and EEG decreased by 16.22% and 19.35%, respectively, from low to heavy urbanized areas. Moreover, SOC decreased from 39.9 to 1.4 mg g^-1, while EEG/SOC and TG/SOC increased by approximately 17% and 34%, respectively, indicating the significant contribution of GRSP to the SOC pool. Pearson and redundancy analysis showed that GRSP was positively correlated with SOC, phosphorus, nitrogen, vegetation richness, and tree height, but negatively correlated with pH, bulk density, and impervious area. The partial least squares path model demonstrated that urbanization affected soil properties, forest characteristics, and land use factors, resulting in GRSP changes. This study clarifies the key factors of urbanization that affect GRSP and provides insight for urban greenspace soil improvement from the new perspective of enhancing the GRSP content.

Effects of vegetation and terrain changes on spatial heterogeneity of soil C-N-P in the coastal zone protected forests at northern China

Soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) are important indicators reflecting soil quality, and they can be used to effectively evaluate the effect of soil remediation. Many studies have evaluated the content of SOC, TN and TP in different ecosystems. However, after constructing protected forests for ecological restoration in the ecologically fragile coastal zone, the spatial distribution and influencing mechanism of SOC, TN and TP content is still uncertain. In this study, the spatial heterogeneity and influencing factors of SOC, TN and TP in surface (0-20 cm) soil were analyzed by traditional analysis and geostatistics. A total of 39 soil samples were collected under the coastal zone protected forest types including Quercus acutissima Carruth (QAC), Pinus thunbergii Parl (PTP), mixed PTP and QAC (QP) and Castanea mollissima BL (CMB) in the coastal zone protected forests in northern China. The results show that SOC, TN and TP content were defined as moderate variation, and they also show significant changes under different protected forest types (P < 0.05). The semivariance results indicate that SOC, TN and TP all exhibited strong spatial dependence class, with Range of 224 m, 229 m and 282 m respectively, which were more than the sampling scale of 200 m. The spatial prediction results showed that SOC, TN and TP content all appear in large areas of extremely low value in CMB, and its cross validation results showed that using vegetation and terrain factors as covariates in the spatial prediction of SOC, TN and TP can improve the prediction accuracy. The results of correlation analysis showed that the influencing factor for SOC and TN, and TP were NDVI and topographical changes, respectively. In general, vegetation and terrain factors as auxiliary factors can improved the accuracy of soil C-N-P spatial distribution prediction after afforestation in coastal zone.