Search

• Wetlands topsoil can sequester more C than farmlands and forestlands. • Soil water content was important factor affecting microbial abundance and enzyme activity. • Higher soil water content can increase soil C storage by stimulating C fixing enzyme activity. • Soil C storage were negatively driven by soil bacterial abundance under different land-use. Soil carbon storage plays a crucial role in mitigating the climate warming and is significantly impacted by the land use pattern. To better understand soil carbon stability and the underlying microbial mechanism, based on qPCR, ELISA, and fluorescence techniques, the effects of land use changes on soil microbial abundance, enzyme activity, and soil carbon storage were investigated in the complex ecosystems of cropland, forestland, and wetland along three typical river (Wolvlan river, Bielahong river, Wusuli river) basin in Sanjiang Plain, China. We hypothesised that croplands soil carbon storage is lower than forestlands and wetlands, which is regulated by soil water content, microbe and enzyme activity. Results showed that topsoil TC in wetlands (85.54 mg·g−1) was higher than those in croplands (33.46 mg·g−1) and forestlands (46.13 mg·g−1). Topsoil water content increased 73.90 % and 71.64 % in wetlands than in farmland and forestlands, respectively. Additionally, reclamation diminished topsoil carbon storage. In croplands, soil dissolved organic carbon, microbial biomass carbon, microbial biomass nitrogen, available nitrogen, bacterial abundance, and soil enzymes (i.e., RubisCO, β-glucosidase, cellobiohydrolase, 1,4-N-acetylglucosaminidase and acid phosphatase) activities were substantially lower than those in wetlands. Structural equation modeling showed that soil carbon storage was positively related to soil water content and RubisCO activity but negatively related to soil bacterial abundance. Soil water content had the largest standardised total positive effect coefficient and indirectly influenced soil carbon storage by affecting RubisCO activity. Soil depth had a greater effect on SMC, hydrolase and site had a greater effect on bacteria and fungi abundance. The results highlight that the alteration of carbon fixing enzyme activity by soil water content is a crucial mechanism for soil carbon sequestration in response to land-use change. Management strategies that conserve natural wetlands, increase soil water content and minimize land disturbance would be effective in maintaining soil carbon stocks. [ABSTRACT FROM AUTHOR]

• Soil water content (SWC) is determined as the outcomes of land use changes. • A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. • The shifts in organic matter sources over time and distinct mechanisms of degradation and preservation of SOM were underscore in wetlands following land use changes. Land use change in wetlands leads to significant losses of soil organic matter (SOM). Stable carbon (C) and nitrogen (N) isotopes offer insights into changes in C3/C4 vegetation, SOM sources, and decomposition processes. Yet, predicting the spatial–temporal dynamics of SOM contents and isotopes under land use changes remains challenging. This study delves into the effects of land use changes on soil organic carbon (SOC), total nitrogen (TN), δ13C and δ15N values, and soil physico-chemical properties and lignin phenols. Our results highlight the significance of soil water content (SWC) in determining the outcomes of land use changes. The conversion of wetland to cropland, forestland and construction land, led to notable reductions in SOC contents (8.71–56.33 %), and TN contents (7.87–37.12 %). Wetland conversion resulted in an enrichment of 13C and 15N abundance, with wetlands exhibiting the lowest δ13C (−25.57 to –22.89 ‰) and δ15N (2.66 to 6.67 ‰) values. A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. Key parameters, including bulk density (BD), C:N, the acid-to-aldehyde of vanillyl ((Ad/Al)v), lignin content (Λ 8), and total phosphorus (TP), were identified as influencing factors for both SOC and TN contents. When evaluating δ13C values, the most influential factors included silt, C:N, SOC, sand, and BD. These indicate the importance of soil chemical group (from 41 % to 21 %) in elucidating δ13C values declined, while lignin group's (from 9 % to 28 %) importance increased from topsoil to subsoil. The acid-to-aldehyde of syringyl ((Ad/Al)s), Λ 8 , C:N, BD and the cinnamyl-to-vanillyl ratio (C/V) were identified as the primary factors influencing δ15N values, with chemical group accounting for 36 % and lignin group for 48 % in topsoil, while physical group dominated 42 % in subsoil. Our findings underscore the shifts in SOM sources and distinct mechanisms of degradation/preservation of SOM following land use changes. [ABSTRACT FROM AUTHOR]

• The agri-horticulture (AH) system exhibited the highest Carbon management index (CMI). • CMI increased at higher altitude in dry temperate Himalayas. • Passive carbon pool was higher under AH system than agriculture (A) by 16.2 %. • The highest Lability index was reported in A indicating lower stability than AH. Globally, land use management consistently changes the dynamics of labile and recalcitrant (non-labile) soil organic carbon (SOC) fractions. However, the effects vary depending on altitude, soils and management practices. The dry temperate north-western Himalayan region is characterized by the presence of diverse land use systems, however the comparison of these SOC fractions across various land use regimes is not well-documented in the area. This study reported the differences in labile and non-labile SOC fractions under the major land-use systems viz., Agriculture (A), Horticulture (H), Agri-horticulture (AH) and Grass fallow (OF) land in various soil-depth classes (0–20, 20–40 and 40–60 cm) along three altitudinal ranges i.e., 2200–2500, 2500–2800 and 2800–3100 m above mean sea level (m. a.m.s.l.) across the dry temperate region of north-western Himalayas. The lability index and carbon management index of the soils were determined using these fractions. The analysis affirmed the highest accumulation of active fractions viz., very-labile (2.66 mg g−1), and labile C fraction (2.55 mg g−1) under AH, while the highest recalcitrant fractions i.e., less-labile (3.34 mg g−1) and non-labile C fraction (7.46 mg g−1) was documented under H. Moreover, the Carbon Management Index (CMI) followed the order of AH > A > H > OF. All the SOC fractions, pools and CMI significantly increased with increase in altitude but declined at lower soil depths. The Pearson correlation analysis revealed that the CMI was positively correlated with the active C fractions i.e. C frac 1 (r = 0.96; p < 0.05) and C frac 2 (r = 0.9; p < 0.05). The higher active carbon pool and CMI (6.9–16 % and 4.1–15.7 % higher, respectively) under the AH may be due to the greater organic matter inputs to the soil under the agri-horticulture land use system. The findings of this study suggest that the soil can be managed sustainably by the adoption of stable land use systems such as agri-horticulture (owing to their higher CMI values) in the highly fragile dry temperate ecosystem of the north-western Himalayas. [ABSTRACT FROM AUTHOR]