Contribution of the vertical movement of dissolved organic carbon to carbon allocation in two distinct soil types under Castanopsis fargesii Franch. and C. carlesii (Hemsl.) Hayata forests

International audienceAbstractKey messageThe vertical transport of dissolved organic carbon (DOC) is an important determinant of carbon distribution across a soil profile. The transport of DOC down a soil profile can be largely influenced by incoming DOC and soil organic carbon (SOC) levels, which insulate DOC from adsorption processes regulated by soil texture and Fe/Al mineralogy.ContextUncertainties about how soil properties affect DOC transport through the soil profile require study because soils can differ strongly with respect to texture or Fe/Al mineralogy and yet retain similar quantities of DOC.AimsThis study aimed to assess the role of incoming DOC and native SOC in regulating DOC migration in soils and investigate the contribution of DOC movement to SOC allocation.MethodsWe leached a standard DOC solution extracted from Castanopsis carlesii litter through two distinct soil types, using two leaching strategies: single leaching and sequential leaching. The two soil types under a natural Castanopsis carlesii (Hemsl.) Hayata forest and a natural Castanopsis fargesii Franch. forest, respectively, differ strongly with respect to soil texture, Fe/Al oxide abundances, and SOC nature.ResultsWith single leaching, where each of six soil layers making up an entire 0–100-cm soil depth profile received single doses of standard DOC solution, deeper soil layers retained more DOC than upper soil layers, with native SOC largely masking the effects of soil texture and Fe/Al mineralogy on DOC migration. Following sequential leaching, where a sixfold larger amount of standard DOC solution sequentially percolated through the six soil layers, the upper soil layers generally retained more DOC than deeper layers. Nevertheless, in sequential leaching, desorption-induced transfer of carbon from upper soil layers to deeper soil layers resulted in greater total carbon retention than in single leaching.ConclusionForest subsoils (40–100 cm) are well below C saturation, but DOC vertical movement from top soils only transfers limited organic carbon to them. However, DOC vertical movement may greatly alter SOC allocation along the top soil profile (0–40 cm), with part of outer sphere native SOC displaced by incoming DOC and migrating downwards, which is a natural way to preserve SOC

The Different Factors Driving SOC Stability under Different N Addition Durations in a <i>Phyllostachys edulis</i> Forest

As one of the most widespread driving forces in the world, atmospheric nitrogen (N) deposition can significantly alter the carbon cycling of ecosystems. In order to understand how N deposition regulates soil organic carbon (SOC) dynamics and its underlying mechanisms, a 7-year N addition experiment was set in a Phyllostachys edulis forest with three N addition levels (+0, +20, and +80 kg N hm−2 year−1) to evaluate the effects of N addition on the concentration and stability of SOC fractions in the third, fifth, and seventh years. The results are as follows: (1) short-term (third year) N addition markedly increased SOC stability by decreasing the concentration of particulate organic carbon (POC) and increasing the mineral-associated organic carbon (MAOC); longer duration of N addition (5 and 7 years) had an insignificant effect on SOC stability and fractions, suggesting that the effects of N deposition on the SOC stability varied under different duration regimes; (2) N addition did not significantly affect microbial community composition while increasing the ratio of fungi to bacteria (F:B) in the seventh year, and microbial biomass carbon (MBC) and carbon use efficiency (CUE) were significantly increased in the short-term (third year) high N addition regime and enzyme activity was significantly increased in the seventh years’ high N addition regime; (3) variation partitioning analysis and multiple regression analysis showed that SOC fractions are mainly regulated by CUE and MBC under short-term N addition, while enzyme activity was mainly regulated under the longer duration of N addition. Our results show that SOC stability was more sensitive in the short term, and the role of microbial characteristics varied under different N addition durations in the P. edulis forests. Overall, our findings provide a new perspective for the responses of the SOC pool to N deposition and contribute to predicting SOC dynamics in terrestrial ecosystems under future climate change

Nitrogen along the Hydrological Gradient of Marsh Sediments in a Subtropical Estuary: Pools, Processes, and Fluxes

Knowledge on the distribution of nitrogen (N) pools, processes, and fluxes along hydrological gradients provides a comprehensive perspective to understand the underlying causal mechanisms in intertidal flats, and thus improve predictions and climate adaptation strategies. We used a space-for-time substitution method to quantify N pools, processes, and fluxes along a hydrological gradient. Further, we linked N pools and processes and investigated not only surface but also subsurface sediments. Our results showed a gradual decrease in total N (TN) and mineralization rates (PNmin), but an increase in potential rates of nitrification (PNR) and denitrification (PDNR) under an elevated hydrological gradient, except for TN and PNmin in the subsurface sediment, which accumulated on the interaction zone between the high and middle tidal flats. Most sedimentary ammonium N (NH4+) and nitrate N (NO3&minus;) concentrations were similar; however, NH4+ accumulated on the subsurface of the middle tidal flat. NO3&minus; fluxes (from &minus;0.54 to &minus;0.35 mmol m&minus;2 h&minus;1) were uptake fluxes in the intertidal flats, but NH4+ fluxes (&minus;2.48&ndash;3.54 mmol m&minus;2 h&minus;1) changed from uptake to efflux in the seaward direction. Structural equation modeling of the effects of inundation frequency, underground biomass, total carbon (TC), electrical conductivity (EC), and clay proportion on the N processes revealed that these accounted for 67%, 82%, and 17% of the variance of PDNR, PNmin, and PNR, respectively. Inundation frequency, underground biomass, TC, EC, and PNmin effects on N pools accounted for 53%, 69%, and 98% of the variance of NH4+, NO3&minus;, and TN, respectively. This suggests that future sea level rise may decrease N storage due to increase in coupled nitrification&ndash;denitrification and decrease in N mineralization, and the NH4+ flux may change from sink to source in intertidal ecosystems

Keystone Soil Microbial Modules Associated with Priming Effect under Nitrogen- and Glucose-Addition Treatments

The priming effect (PE) is important for understanding the decomposition of soil organic matter (SOM) and forecasting C-climate feedback. However, there are limited studies on microbial community-level properties and the keystone taxa involved in the process. In this study, we collected soil from a subtropical Phyllostachys edulis forest undergoing long-term N-addition and conducted an incubation experiment to evaluate the effects of single and repeated addition of 13C-labeled glucose. Our results demonstrated that previously N-fertilized soil had a smaller cumulative PE compared with that of the control (11% average decrease). This could be primarily explained (26%) by the lower abundance of bacterial r-strategy group members (B_mod#2, constituting Proteobacteria, Firmicutes, and Actinobacteria phyla) under N-addition treatments. A single C-addition induced a greater PE than that of repeated C-additions (2.66- to 3.11-fold). Single C addition led to greater C to N ratios of microbial biomass and fungi to bacteria, positively impacting cumulative PE, indicating that the shifts in fungal/bacterial dominance play an important role in regulating PE. Moreover, a saprophytic taxa group (F_Mod#3, primarily composed of the phyla Ascomycota) explained 62% of the differences in cumulative PE between single and repeated C-additions. Compared with repeated C-additions, a greater abundance of B_Mod#2 and F_Mod#3, as well as C-related hydrolase activity, was observed under single C-addition, inducing greater cumulative PE. Therefore, sufficient C may facilitate the proliferation of r-strategy bacterial taxa and saprophytic fungal taxa, thereby increasing SOM decomposition. Our findings provide novel insights into the relationship between microbial community-level properties and PE

Nitrogen Addition Affects Soil Respiration Primarily through Changes in Microbial Community Structure and Biomass in a Subtropical Natural Forest

Forest soil respiration plays an important role in global carbon (C) cycling. Owing to the high degree of C and nitrogen (N) cycle coupling, N deposition rates may greatly influence forest soil respiration, and possibly even global C cycling. Soil microbes play a crucial role in regulating the biosphere&#8722;atmosphere C exchange; however, how microbes respond to N addition remains uncertain. To better understand this process, the experiment was performed in the Castanopsis kawakamii Hayata Nature Reserve, in the subtropical zone of China. Treatments involved applying different levels of N (0, 40, and 80 kg ha&#8722;2 year&#8722;1) over a three-year period (January 2013&#8722;December 2015) to explore how soil physicochemical properties, respiration rate, phospholipid fatty acid (PLFA) concentration, and solid state 13C nuclear magnetic resonance responded to various N addition rate. Results showed that high levels of N addition significantly decreased soil respiration; however, low levels of N addition significantly increased soil respiration. High levels of N reduced soil pH and enhanced P and C co-limitation of microorganisms, leading to significant reductions in total PLFA and changes in the structure of microbial communities. Significant linear relationships were observed between annual cumulative respiration and the concentration of microbial biomass (total PLFA, gram-positive bacteria (G+), gram-negative bacteria (G&#8722;), total bacteria, and fungi) and the microbial community structure (G+: G&#8722; ratio). Taken together, increasing N deposition changed microbial community structure and suppressed microbial biomass, ultimately leading to recalcitrant C accumulation and soil C emissions decrease in subtropical forest

Land use change exerts a strong impact on deep soil C stabilization in subtropical forests

201

Keystone bacterial functional module activates P-mineralizing genes to enhance enzymatic hydrolysis of organic P in a subtropical forest soil with 5-year N addition

Microorganisms play an integral role in driving phosphorus (P) transformation in forest soils; however, studies on soil P cycling and the molecular mechanisms of microbes activated in response to elevated nitrogen (N) deposition are limited. In this study, we conducted a multilevel field N enrichment experiment in a subtropical P-deficient Moso bamboo (Phyllostachys heterocycla) system to evaluate the microbial ecological traits of P transformation (e.g., organic P mineralization and inorganic P solubilization) over three consecutive years. N addition significantly decreased available and organic P levels in the soil and increased the microbial biomass C:P and N:P ratios, indicative of severe microbial P limitation. Consequently, N addition increased the absolute abundance of P starvation response regulation genes (phoU and phoR), which further induced an increase in organic P mineralization (phoN, phoD, appA), but not that of inorganic P solubilization genes (ppx and gcd). This suggests that microbes enhance P availability by organic P mineralization rather than inorganic P solubilization to ameliorate reduced P availability. Furthermore, a bacterial functional module (B_Mod#0) consisting of Proteobacteria, Actinobacteria, and Firmicutes accounted for more than 60% of the changes in the abundance of genes responsible for organic P-mineralization in the soil, suggesting that B_Mod#0 acts as a keystone phylotype in enhancing functional P-cycling potential. This study provides novel insights into microorganism-driven P cycling in P-deficient forest soils with N addition