High levels of soil calcium and clay facilitate the recovery and stability of organic carbon: Insights from different land uses in the karst of China.

Soil organic carbon (SOC) is a crucial medium of the global carbon cycle and is profoundly affected by multiple factors, such as climate and management practices. However, interactions between different SOC fractions and land-use change have remained largely unexplored in karst ecosystems with widespread rock outcrops. Owing to the inherent heterogeneity and divergent response of SOC to land-use change, soil samples with close depth were collected from four typical land-use types (cropland, grassland, shrubland, and forestland) in the karst rocky desertification area of China. The aim of this study was to explore the responses of SOC dynamics to land-use types and underlying mechanism. The results showed that land-use type significantly affected SOC contents and its fractions. Compared with cropland, the other three land uses increased the total organic carbon (TOC), microbial biomass carbon (MBC), and non-labile organic carbon (NLOC) contents by 6.11-129.44%, 32.58-173.73%, and 90.98-347.00%, respectively; this demonstrated that a decrease in both labile and recalcitrant carbon resulted in SOC depletion under agricultural land use. Readily oxidized organic carbon (ROC) ranged from 42 to 69%, accounting for almost half of the TOC in the 0-40-cm soil layer. Cropland soil showed significantly higher ROC:TOC ratios than other land-use types. These results indicated that long-term vegetation restoration decreased SOC activity and improved SOC stability. Greater levels of soil exchangeable calcium (ECa) and clay contents were likely responsible for higher stabilization and then accumulation of SOC after vegetation restoration. The carbon pool index (CPI) rather than the carbon pool management index (CPMI) exhibited consistent variation trend with soil TOC contents among land-use types. Thus, further study is needed to validate the CPMI in evaluating land use effects on soil quality in karst ecosystems. Our findings suggest that land-use patterns characterized by grass or forest could be an effective approach for SOC-sequestration potential and ensure the sustainable use of soil resources in the karst area.

Investigating the recovery in ecosystem functions and multifunctionality after 10 years of natural revegetation on fly ash technosol.

Technogenic soil (technosol) developed from coal fly ash (FA) landfilling has been considered a critical environmental problem worldwide. Drought-tolerant plants often naturally grow on FA technosol. However, the impact of these natural revegetations on the recovery of multiple ecosystem functions (multifunctionality) remains largely unexplored and poorly understood. Here we assessed the response of multifunctionality, including nutrient cycling (i.e., carbon, nitrogen, and phosphorus), carbon storage, glomalin-related soil protein (GRSP), plant productivity, microbial biomass carbon (MBC), microbial processes (soil enzyme activities), and soil chemical properties (pH and electrical conductivity; EC) to FA technosol ten years' natural revegetation with different multipurpose species in Indo-Gangetic plain, and identified the key factors regulating ecosystem multifunctionality during reclamation. We evaluated four dominant revegetated species: Prosopis juliflora, Saccharum spontaneum, Ipomoea carnea, and Cynodon dactylon. We found that natural revegetation initiated the recovery of ecosystem multifunctionality on technosol, with greater recovery under higher biomass-producing species (P. juliflora and S. spontaneum) than lower biomass-producing ones (I. carnea and C. dactylon). The individual functions (11 of the total 16 variables) at higher functionality (70 % threshold) also exhibited this pattern among revegetated stands. Multivariate analyses revealed that most of the variables (except EC) significantly correlated with multifunctionality, indicating the capability of multifunctionality to consider the tradeoff between individual functions. We further performed structural equation modeling (SEM) to detect the effect of vegetation, pH, nutrients, and microbial activity (MBC and microbial processes) on ecosystem multifunctionality. Our SEM model predicted 98 % of the variation in multifunctionality and confirmed that the indirect effect of vegetation mediated by microbial activity is more important for multifunctionality than their direct effect. Collectively, our results demonstrate that FA technosol revegetation with high biomass-producing multipurpose species promotes ecosystem multifunctionality and emphasizes the significance of microbial activity in the recovery and maintenance of ecosystem attributes.

Effects of hillslope position on soil water infiltration and preferential flow in tropical forest in southwest China.

The hillslope is an essential natural spatial gradient that influences hydrological processes by affecting water distribution, surface flow, soil erosion, and groundwater recharge. To date, few studies have addressed only the hydrological processes of tropical forest hillslopes. To reveal the effect of hillslope on soil hydrological functioning-including water distribution and exchange, infiltration capacity, and flow behaviour-we conducted 36 field infiltration and nine dye-tracer investigations of different hillslope locations in the natural rainforest of Xishuangbanna, southwest China. The soil physical properties-including soil noncapillary and total porosity, saturated water capacity, and field water capacity-decreased with decreasing elevation from hilltop to middle slope and the valley bottom. The water infiltration capacity-including the initial infiltration rate, saturated soil hydraulic conductivity, and average infiltration rate-decreased from the hilltop to the valley bottom. Preferential flow dominated soil water movement more in the upper locations than in the valley bottom. The infiltration capacity parameters and preferential flow were significantly correlated with soil water content, noncapillary and total porosity, root biomass, and termite holes. These results indicated that along with the soil physical properties, root systems, animal activity, cracks, and stones affected the soil infiltration capacity and preferential flow. Differences in the hydraulic processes of each hillslope position contributed to the redistribution, transportation, and storage of surface and belowground water, resulting in differing availabilities of soil water resources and utilisation by plants. The findings of this study can help understand eco-hydrological processes in the context of water resources management in tropical mountain ecosystems.

A global review of rubber plantations: Impacts on ecosystem functions, mitigations, future directions, and policies for sustainable cultivation.

The growing global need for latex is driving rubber plantation (RP) expansion since the last century, with >2 Mha of cultivation area being established in the last decade. Southeast Asia is the hotspot for rubber cultivation at other land-use costs. Although rubber cultivation has improved the economic status of farmers, it has altered the habitat's ecology and ecosystem functions (EF). However, studies on the impacts of RP on EF are limited, and a clear overview is not available. To bridge this gap, we conducted an inclusive review of the EF of RP, including soil carbon storage, aboveground biomass (AGB) and belowground biomass (BGB), litter production and decomposition, respiration, and biodiversity (plants, animals, soil fauna, and microbes). We compared the EF in RP (monoculture) with those in forests because the conversion of forests to RP is prevalent in the tropics and because most RP studies used forests as reference ecosystems. We found RP generally have lower EF than forests. The impacts of RP on some EF are more severe (e.g., AGB, BGB, and plant diversity), causing decreases of >55%, and the effects are consistently negative irrespective of plantation age. However, including agroforestry or polyculture, integrated pest management, cover cropping, mulching, and composting can improve the EF in RP to some extent. We highlighted research gaps, particularly substantial research gaps concerning the influence of plant diversity treatments (i.e., agroforestry) performed in RP on EF. Additionally, more empirical data on the significance of spatial and temporal levels are required, such as how the impact on EF could vary with climate and RP age, as we showed some examples where EF differs spatially and temporally. More importantly, further research on plantation management to offset EF losses is needed. Finally, we emphasized knowledge gaps and suggested future directions and policies for improving EF in RP.

Influence of sulfur amendments on heavy metals phytoextraction from agricultural contaminated soils: A meta-analysis.

Heavy metal pollution is becoming recurrent and threatens biota biosafety in many agricultural fields. Diverse solutions explore the application of amendments to enable remediation. Sulfur represents a nonmetallic chemical element that actively affects heavy metals phytoextraction, and promotes and alternatively mitigates soil functions. In this study, we conduct a meta-analysis to synthesize the current knowledge on the influence of sulfur amendments on plants heavy metals uptake from contaminated soil media. Random-effects model was used to summarize effect sizes from 524 data points extracted from 30 peer reviewed studies. The phytoextraction of cadmium, chromium and nickel were 1.6-, 3.3-, and 12.6-fold, respectively, higher when sulfur amendment was applied; while copper uptake was 0.3-fold lower. Irrespective of the sulfur type, heavy metal extraction increased with the raising sulfur stress. Individual organs showed significant differences of heavy metal uptake between sulfur applied and non-sulfur treatments, and combined organs did not. The heavy metals uptake in leaves and roots were higher in sulfur applied than non-sulfur applied treatments, while those in grain, husk, and stalks were lower. The heavy metals phytoextraction (response ratio) followed the order roots > leaves > stalk > grain > husk. Moreover, heavy metals uptake was 2-fold higher in the sulfur applied than the non-sulfur treatments under ideal (5.5-8) and alkaline conditions (8-14), and 0.2-fold lower under acidic pH (1-5.5). Cadmium, manganese and nickel, and chromium were the most extracted under sulfur application by Vicia sp., Sorghum sp. and Brassica sp., respectively; while chromium, manganese, and iron were the most uptake without sulfur amendments by Oryza sp., Zea sp. and Sorghum sp., respectively. Our study highlights that the influence of sulfur on heavy metal phytoextraction depends on the single or combined effects of sulfur stress intensity, sulfur compounds, plant organ, plant type, and soil pH condition.

Soil quality assessment of different Hevea brasiliensis plantations in tropical China.

Land degradation is a global problem caused by improper agricultural practices. In tropical China, the rubber (Hevea brasiliensis) plantations are predominantly practiced on forest-cleared lands, considering their sustainable land management potential compared to annual cropping. However, all rubber plantations may not have similar land management capacity. Soil quality index (SQI) can reveal the overall soil status with a single score, which is an efficient tool to evaluate the soil quality of each category of rubber plantations. We investigated 23 soil physical and chemical parameters of three categories of rubber plantations and a primary rainforest, and derived SQI based on these parameters. Soil samples were collected from a rubber monoculture (RM), a rubber-Camellia sinensis agroforestry (RT), a rubber-Dracaena cochinchinensis agroforestry (RD), and a primary rainforest (RF). The results showed that the SQI value of the RM decreased by 15.50% compared to the RF, with a significant degree of soil nutrient loss (18.90%). This indicates that monocultural rubber cultivation is causing land degradation to some extent. However, the SQI was significantly enhanced by rubber-based agroforestry practices (25.30% by RT and 33.10% by RD) compared to the RM, suggesting that polyculture practices are suitable to recover the soil quality in degraded agricultural lands. Moreover, the chemical parameters contributed more to the SQI than did the physical parameters, indicating that nutrient management is important in soil quality recovery. Overall, our results suggest that agroforestry should be preferred over monoculture in the rubber plantations for sustainable land management in tropical China.

Jungle rubber facilitates the restoration of degraded soil of an existing rubber plantation.

Conversion of forest to rubber plantation is one of the most common land-use change in the humid tropical region. It is one of the fastest expanding farms that lead to various socioenvironmental issues. We investigated the effect of this land-use change on soil physico-chemical properties by surveying different succession stage rubber plantations, including monoculture and a mixture derived by mixing jungle rubber and a reference tropical rainforest. We also assessed the impact on stoichiometric ratios and allocation relationships of soil carbon (C), nitrogen (N), and phosphorus (P). Our results demonstrated that conversion of tropical rainforest to rubber monoculture resulted in serious soil degradation, with a lower level of water content, water holding capacities, total porosity, pH, and soil nutrients, and a higher level of soil bulk density. However, after transforming a rubber monoculture into a jungle rubber, the concentrations of soil total C, N, P, Ca, and Mg significantly increased, by 28%, 24%, 23%, 17%, and 39%, respectively. Meanwhile, soil salinity declined by 15%. Jungle rubber also exerted some desirable effects on soil physical properties, such as decreased soil bulk density, increased field capacity and non-porosity by 6%, 2%, and 33%, respectively. Like other tropical regions, soils in the present study areas are mainly under P limitation, but jungle rubber increased soil P turnover and thereby increases P availability. In conclusion, jungle rubber correcting the soil degradation resulted from rubber plantation on tropical forest soil. Given the improvements in soil quality, constructing multiple-strata and multi-species rubber agroforestry (e.g., jungle rubber) can be a promising approach to facilitate the restoration of the existing monoculture rubber plantations.

Litterfall biomass and nutrient cycling in karst and nearby non-karst forests in tropical China: A 10-year comparison.

Litterfall helps maintaining primary production and nutrient cycling in forest ecosystems. However, few studies have investigated long-term characteristics of litterfall in tropical karst and non-karst forests co-occurring in the same region. A 10-year comparative study was conducted to estimate the biomass, litter accumulation, turnover rate, nutrient return and nutrient use efficiency associated with litterfall in a karst forest (KF) and a nearby non-karst forest (NKF) in northern tropical China. Significant spatial-temporal variation was observed in monthly and annual litterfall biomass in the two forests. Annual mean litterfall biomass in KF (9.75 Mg ha-1 year-1) was obviously lower than that in NKF (10.49 Mg ha-1 year-1). The litterfall biomass in NKF was significantly correlated with maximum air temperature, wind speed and total solar radiation, whereas that in KF was significantly correlated with relative humidity, wind speed and low temperature. Average stand litter in KF (2.92 Mg ha-1 year-1) was significantly higher than that in NKF (2.38 Mg ha-1 year-1). Stand litter mostly occurred during the cool and dry season, which coincided with litterfall input and exhibited bimodal pattern. Turnover rate was 1.3 time higher in NKF than in KF, suggesting that litter decomposed slowly in karst habitats. Distinct temporal dynamic and significant differences were observed in chemical composition of litterfall between KF and NKF. Total amounts of C, P, K and total nutrients returned to the topsoil in KF were significantly lower than those in NKF. The KF exhibited relatively high P and K use efficiency because of their low availability in karst soils. Compared with the non-karst habitat, the tropical karst habitats are more likely to develop a plant community with certain nutrient concentrations of litterfall and with a nutrient cycling mechanism that is well-adapted to harsh and heterogeneous condition.

Lateral flow between bald and vegetation patches induces the degradation of alpine meadow in Qinghai-Tibetan Plateau.

Bald patches (BPs) are known to accelerate and simultaneously mitigate the process of desertification. However, the mechanisms of these two synchronous actions are little studied in high desert and cold systems; and the incidence of BPs on alpine meadows degradation in Qinghai-Tibetan Plateau (QTP) of China is still unavailable. This study first aims to investigate the soil properties and the erodibility of the system BPs-VPs at the Beiluhe basin in QTP. Then, we adopted dye tracer and HYDRUS-2/3D methods to interpret the water infiltration patterns from point scale to slope scale. The results show that the mattic epipedon layer on the top soil (0-20 cm) of VPs directly reduced the impact of raindrops on alpine meadow; and the adhesion of root system in VPs prevented the soil particles from stripping and washing away by runoff. The soil particles in BPs were easily eroded by rainfall, lowering the ground level of BPs relative to the ground level of VPs. The two patches therefore alternated to form an erosion interface where marginal meadow was likely detached by raindrops, and washed away through runoff. The saturated hydraulic conductivity (Ks) of surface soil (0-10 cm) was 124% higher in BPs than the VPs. Thereby, BPs caused a high spatial variation of infiltration and runoff in QTP. Moreover, this difference in Ks between the two patches conducted to a lateral flow from BPs to VPs, and to soil layers with different water contents. These findings highlight that the water flow features can potentially disturb the processes of freezing-thawing, frost heaves, and thaw slump; and accelerate the alpine meadow degradation. Therefore, land cover such as crop and vegetation should be applied over the bare soil surface to prevent the degradation of alpine meadow.

Decay and erosion-related transport of sulfur compounds in soils of rubber based agroforestry.

Elemental sulfur is intensively used to control weeds and rubber leaf diseases. However, the mechanisms contributing to elemental sulfur dissipation and decay (hereafter decay) in rubber agroforestry remains unclear. This study relates hydrological processes such as runoff and soil loss to the changes in soil total sulfur (Stot) and sulfate (S-SO4) in typical hillslope rubber agroforestry intercropped with cocoa in Xishuangbanna. The elemental sulfur decay kinetics were studied at two slopes (top and bottom) and three agrosystems (weed, no-weed and mixed). The results show that soil moisture and hydraulic conductivity was uniformly distributed in the experimental rubber agroforestry settings. Higher soil loss and runoff occurred in the bottom slope than the top slope, and in no-weed agrosystem than the herbaceous agrosystems (weed and mixed). The soil loss was mainly driven by runoff. Moreover, Stot and S-SO4 in runoff water were higher in weed agrosystem than no-weed agrosystems. Soil Stot best fit a two-compartments kinetics model, with lower kinetic rates in elemental sulfur applied treatments than in the no-added elemental sulfur treatments, particularly for the weed agrosystem. The soil Stot dissipation time 50% (DT50) was 10-14 times higher in top slope than bottom slope; but 4 and 20 times higher in mixed and no-weed agrosystems, respectively, compared to the weed agrosystem. The soil Stot and S-SO4 contents negatively correlated with soil microbial respiration (CO2 efflux), indicating an adverse influence of elemental sulfur on soil microbial activity. In short, elemental sulfur decay and its S-SO4 transformation depended on soil moisture, runoff, soil erosion and soil CO2, which are in turn affected by slope and agrosystem. This study not only clarifies the mechanisms of elemental sulfur dissipation and decay for its use as an environmental friendly agrochemical; but it also provides information to understand the contribution of runoff and soil loss on these mechanisms in rubber agroforestry.

Spatial distribution patterns of rock fragments and their underlying mechanism of migration on steep hillslopes in a karst region of Yunnan Province, China.

In mountainous areas, rock fragments (RFs) are a common feature on the soil surface and in topsoil. Few studies, however, have investigated the spatial distribution of RFs and the relevant mechanisms underpinning their distribution on steep hillslopes, especially in karst regions. We have collected and measured the RF cover, size, and content at the soil surface and within the topsoil of secondary forest, man-made forest, and non-forest land hillslopes in a karst region in Yunnan Province, southwest China. The results revealed no significant relationships between slope position and mean total RF coverage, median diameter (D50), and mean total volumetric RF in topsoil within the three karst hillslopes covered by different types of vegetation. A limited effect of vegetation on the spatial distribution of RFs on the hillslopes was identified. However, the variation in RFs in the topsoil between the top and bottom slopes was greater than that at the surface between the top and bottom slopes, implying that underground leakage was greater than surface runoff.

Humus soil as a critical driver of flora conversion on karst rock outcrops.

Rock outcrop is an important habitat supporting plant communities in karst landscape. However, information on the restoration of higher biotic populations on outcrops is limited. Here, we investigated the diversity, biomass changes of higher vascular plants (VP) and humus soil (HS) on karst outcrops during a restoration process. We surveyed VP on rock outcrops and measured HS reserved by various rock microhabitats in a rock desertification ecosystem (RDE), an anthropogenic forest ecosystem (AFE), and a secondary forest ecosystem (SFE) in Shilin County, southwest China. HS metrics (e.g. quantity and nutrients content) and VP metrics (e.g. richness, diversity and biomass) were higher at AFE than at RDE, but lower than at SFE, suggesting that the restoration of soil subsystem vegetation increased HS properties and favored the succession of VP on rock outcrops. There was significantly positive correlation between VP metrics and HS amount, indicating that the succession of VP was strongly affected by availability and heterogeneity of HS in various rock microhabitats. Thus, floral succession of rock subsystem was slow owing to the limited resources on outcrops, although the vegetation was restored in soil subsystem.