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The Grain‐for‐Green Program in China aims to restore sloping karst cropland severely degraded by intensive agriculture to secondary forest. Heterotrophic respiration (Rh) is a major process of carbon release from soils and is associated with the sequestration of soil organic carbon (SOC). However, we still do not have a comprehensive understanding of Rh and what drives it along soil profile horizons during the natural vegetative recovery process in typical karst soils. We investigated the responses of Rh (C release per gram of soil) and specific Rh (C release per gram of SOC) in soil horizons from the soil surface to bedrock at the different vegetation recovery stages in a karst region. Coincident soil microbial properties (e.g., bacterial and fungal abundance, total microbial biomass, potential enzyme activities) and physicochemical properties were quantified. Vegetation restoration after cropland abandonment significantly increased Rh rates due to increased soil nutrients and microbial biomass, bacterial abundances, and hydrolase activities (all, p < 0.05). The rates of Rh enhanced in sloping cropland (SC) from the soil surface to bedrock but reduced in the recovering stages (ASC: abandoned sloping cropland and SF: secondary forest) and primary forest (PF). The specific Rh between SF (1.34 mg CO2‐C g−1SOC d−1) and PF (1.32 mg CO2‐C g−1SOC d−1) were not significantly different but both of them were greater than those in SC (0.79 mg CO2‐C g−1SOC d−1) and ASC (0.8 mg CO2‐C g−1SOC d−1). Soil physicochemical properties and microbial properties explained approximately 48% and 22% of the variations in Rh along vegetation recovery, respectively. Nitrogen to phosphorus stoichiometry exerted the most direct and positive effects on Rh, suggesting the importance of managing soil nutrient status to regulate carbon decomposition during vegetation recovery in karst soils. The increased microbial biomass was the most important microbial factor regulating Rh in later vegetation recovery phases. Our results provide scientific insight into the impact of vegetation restoration on Rh in degraded ecosystems, which is important for reducing carbon loss in karst soils. [ABSTRACT FROM AUTHOR]

Extensive and progressive rock emergence in karst ecosystems may cause localized variations in soil biogeochemical and microbial properties, and thus produce nutrient cycling 'hot spots' that could alter functional responses to perturbation. Here, we investigated the differences between microbial compositions and functions in topsoils at the rock‐soil interface (RSI) compared with adjacent bulk soil along a gradient of increasing human perturbation in the Chinese Karst Critical Zone Observatory. Microbial abundance decreased with increasing perturbation and was higher at the RSI compared to bulk soil. Compared with the bulk soil, C‐cycling and N cycling enzyme activities at the RSI were 72–427% higher, respectively, and those related to N cycling were 72–98% higher, and were greatest in primary forest and abandoned land. Mineral contents explained the large variances in enzyme activities suggesting that mineral availability modified microbial functions for nutrients acquisition in nutrient‐poor karst system. The significantly larger nutrient contents of RSI soil in the primary forest suggest that weathering of the karst rocks in unperturbed environments is the primary source of nutrients, which is driven by microbial enzyme production. The enzyme activities related to C and N cycling were highest in abandoned land, which suggests a rapid switch in microbial function caused by nutrient limitation when cultivation ceased. In conclusion, soil microbial abundance and function next to karst rocks is higher than bulk soils and persists after recovery. This suggests that the potential for long‐term recovery of very degraded karst landscapes is possible after abandonment because microbial functions for C and nutrient cycling persist in RSI 'hot spots'. [ABSTRACT FROM AUTHOR]

Karst topography covers more than 1/3 of the People's Republic of China in area. The porous, fissured, and soluble nature of the underlying karst bedrock (primarily dolomite and limestone) leads to the formation of underground drainage systems. Karst conduit networks dominate this system, and rainfall takes a crucial role on water cycle at China karst area. Nitrogen loss from the karst system is of particular concern, with regard to nutrient use efficiency as well as water quality, as much of the karst system, including steeply sloping terrain, is used for intensive agriculture. We use simulated rainfall experiments to determine the relationship between rainfall and nitrogen loss at typical karst slope land and then estimate nitrogen loss from the karst soil. The results show that both surface runoff and subsurface runoff have a significant linear correlation with rainfall at all studied sites. Subsurface runoff is larger than surface runoff at two karst sites, while the opposite is true at the non-karst site. Exponential function satisfactorily described the correlation between rainfall and nitrogen concentrations in runoff. Nitrates accounted for 60%-95% of the dissolved nitrogen loss (DN, an index of N-loss in this research). The estimated annual N-loss load varies between 1.05 and 1.67 Tg N/year in the whole karst regions of China from 1961 to 2014. Approximately, 90% of the N-loss load occurred during the wet season, and 90% of that passed through the subsurface. Understanding the processes and estimating N-loss is highly valuable in determining long-term soil security and sustainability in karst regions. [ABSTRACT FROM AUTHOR]

Covering extensive parts of China, karst is a critically important landscape that has experienced rapid and intensive land use change and associated ecosystem degradation within only the last 50 years. In the natural state, key ecosystem services delivered by these landscapes include regulation of the hydrological cycle, nutrient cycling and supply, carbon storage in soils and biomass, biodiversity and food production. Intensification of agriculture since the late-20th century has led to a rapid deterioration in Critical Zone (CZ) state, evidenced by reduced crop production and rapid loss of soil. In many areas, an ecological 'tipping point' appears to have been passed as basement rock is exposed and 'rocky desertification' dominates. This paper reviews contemporary research of soil processes and ecosystems service delivery in Chinese karst ecosystems, with an emphasis on soil degradation and the potential for ecosystem recovery through sustainable management. It is clear that currently there is limited understanding of the geological, hydrological and ecological processes that control soil functions in these landscapes, which is critical for developing management strategies to optimise ecosystem service delivery. This knowledge gap presents a classic CZ scientific challenge because an integrated multi-disciplinary approach is essential to quantify the responses of soils in the Chinese karst CZ to extreme anthropogenic perturbation, to develop a mechanistic understanding of their resilience to environmental stressors, and thereby to inform strategies to recover and maintain sustainable soil function. [ABSTRACT FROM AUTHOR]

• Unconfined aquifers have a high aquifer matrix porosity that expedites water flow. • Carbonate bedrock and soil texture will dominate water reallocation. • C/N stoichiometry and pH changes responds to rainfall in Karst slopes. Transportation of carbon and nitrogen during rainfall events is highly important within karst ecosystems and has particular relevance to understand decoupling relation between carbon and nitrogen cycles. The ability of the karst ecosystem to sequester carbon is currently unknown but has potential to be an important residual terrestrial carbon sink. We hypothesis that the runoff response to rainfall events will vary according to whether the slope belongs to karst area, and carbon and nitrogen have imbalanced outputs response to runoff. Hence, a series of in-situ simulated rainfall experiments at three sites (two karst slopes with different carbonate matrixes and one non-karst slope without a carbonate matrix) were conducted in a typical karst zone in southwest China. We measured runoff discharge, and dissolved carbon and nitrogen during the experiments. The results show that the total runoff discharge increased positively with rainfall intensity, with the carbonate matrix making an important contribution to rainfall reallocation. The dissolved carbon loss from runoff plots is much higher than the dissolved nitrogen, with the dissolved C/N revealing the decoupling relation of carbon and nitrogen cycles in this critical karst zone. This initial carbon migrated by runoff at catchment scale is critical to carbon budgets in rivers. Furthermore, nitrogen leaching from the system is crucial as a nitrogen shortage at karst area in southwest China. [ABSTRACT FROM AUTHOR]

• Understanding the mechanisms controlling denitrification rate during cropland revegetation. • From 'leaky' to 'tight' N-cycling along cropland revegetation in karst ecosystem. • Denitrification rate correlated positively to denitrifiers, but negatively to nitrifiers. • PDR in cropland and abandoned cropland was controlled by microbial abundances. • Both PDR and BDR in karst forest soils were controlled by soil phosphorus contents. The revegetation of karst ecosystems in subtropical southwest China that became severely degraded because of intensive agriculture relies on the persistence and recovery of soil functions such as nutrient cycling. Denitrification is a nitrogen (N) removal process that is carried out by nitrifying and denitrifying microorganisms. We still do not have a comprehensive understanding of denitrification and what drives it during revegetation of cropland in degraded karst ecosystems. We used the space-for-time chronosequence method to establish a restoration gradient (sloping cropland, abandoned sloping cropland, and secondary and primary forest) within the Karst Critical Zone Observatory in southwest China. We quantified the abundances of the nitrifier and denitrifier microbial communities and measured the potential denitrification rates (PDR) and basal denitrification rates (BDR) in incubated soils to investigate the denitrification activity in soils at different stages of restoration. The PDR increased through the cropland revegetation phases, and the BDR was lowest in the abandoned sloping cropland (p < 0.05). Both the PDR and BDR were positively correlated with the denitrifier abundances (i.e., nitrite reductase, nirK, and nirS), but were negatively correlated with nitrifier abundances (i.e., ammonia oxidation, AOA, and AOB). The BDR/PDR ratio was 84 % in the sloping cropland but was only 1% in the primary forest, which suggests that the N cycling was inefficient ('leaky') in actively managed farmland soils but was 'tight' in forest soils. The AOB and nirS abundances explained most of the PDR in the sloping cropland and abandoned sloping cropland, while the soil phosphorus (P) contents explained most of the variation in BDR and PDR in land that was forested in later phases of revegetation. Progressive increases in the PDR as the revegetation progressed was directly controlled by the soil available P and total P contents. Available P was also associated with increases in the carbon (C) and N contents, which influenced the BDR indirectly through increased abundances of nirK and nirS. We conclude that revegetation of cropland helped to facilitate 'tight' N-cycling in poor karst soils, and we suggest that judicious P fertilization on sloping cropland would reduce the denitrification activity in degraded ecosystems. [ABSTRACT FROM AUTHOR]