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Knowledge of the effects of pasture degradation on soil organic carbon (SOC) and nitrogen (N) fractions in permafrost soils on the Tibetan Plateau is limited. The aims of this study were (1) to evaluate the changes in SOC and N contents in density fractions under Kobresia pasture due to degradation and (2) to explore the contributions of the changes of SOC and N in density fractions to the changes of SOC and N in whole soil. The impact of Kobresia pasture degradation on SOC and N fractions was investigated in the permafrost region of the Tibetan Plateau. A continuously degraded pasture was identified and classified into three categories of vegetation cover according to their degrees of degradation (i.e., vegetation cover decline from 90% ± 6.6% to 70% ± 8.3% and 45% ± 8.7%). The SOC and N fractions were separated by using the density separation method. The Kobresia pasture degradation significantly decreases SOC and N contents and stocks in soils. The SOC and N contents in the whole soil were positively correlated with the SOC and N contents in the light and heavy fractions (p 0.05). When pasture degraded from vegetation covers 90% to 45%, SOC stock at 0–40-cm soil layer decreased by 28.7% and N stock decreased by 39.2% in the whole soil; 56.6% and 47.6%, respectively, in the light fractions and 14.3% and 40.6%, respectively, in the heavy fractions. The depletion rates of N were higher than those of SOC in the heavy fractions and whole soil. At all sites, more than 80% of the SOC and N stocks were protected in heavy fractions. These results indicate that a decoupling depletion of SOC and soil N appeared with the Kobresia pasture degradation in the permafrost region of the Tibetan Plateau. The Kobresia pasture degradation affects the SOC and N fractions differently and thus regulates soil carbon and N cycling in the permafrost soils on the Tibetan Plateau.

The soil hydrological process plays an important role in transporting and distributing water in soils. However, the correlation between water flow behaviors, soil physical properties, infiltration patterns of soil exhibiting different soil layers, and the variation patterns in water flow behavior in rubber-based agroforestry systems remain poorly understood. Here, we mainly used dye-tracer infiltration to classify water flow behavior and HYDRUS-3D to interpret variation patterns of water flow behaviors in a rubber-based agroforestry system in southwest China. The results showed that (1) the soil of forestry patches under Hevea brasiliensis (HB) and Citrus reticulata (CR) exhibited better soil physical properties than those with no vegetation (NV). Compared with these in the NV, the saturated soil water content and field capacity increased by 15% and 21% in CR, and by 2% and 12% in HB, respectively; while the saturated hydraulic conductivity enhanced by 584% (p HB > NV. Finally, (3) the infiltration flow in soil under HB and CR exhibited a faster downward wetting front and an obvious lateral flow. These infiltration patterns compensated for the insufficiency of soil water in NV and finally caused water redistribution to be more homogenized across the whole soil profile. Therefore, our findings indicated that the rubber-based agroforestry system is a successful approach for conserving water and reducing runoff in tropical regions.

Land use patterns and vegetation coverage in semi-arid areas of the Loess Plateau have undergone great changes due to the implementation of the “Grain for Green” project. The introduction of legume pasture species, such as alfalfa (Medicago sativa L.) and sweet clover (Melilotus officinalis L.), is one of the most efficient methods of vegetation restoration and reconstruction in this region. However, there is a need for an effective assessment of the root system distribution and its interaction with soil after long-term introduction. An experiment involving the introduction of alfalfa and sweet clover on abandoned farmlands was initiated in 2003 to assess the long-term effects. After 17 years, root and soil samples at depths of 0–20 and 20–60 cm were collected to characterize the root biomass, root carbon (C), nitrogen (N), and phosphorus (P), soil microbial biomass carbon (MBC) and nitrogen (MBN), soil organic carbon (SOC), and soil N and P. The results showed that the root biomass density of alfalfa in the 0–20 and 20–60 cm layers (63.72 and 12.27 kg m–3, respectively) were significantly higher than for sweet clover (37.43 and 8.97 kg m–3, respectively) and under natural abandonment (38.92 and 9.73 kg m–3, respectively). The SOC, total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), nitrate-nitrogen (NO3–-N), MBC and MBN in the 0–20 and 20–60 cm layers were higher after alfalfa introduction compared with sweet clover introduction and natural abandonment, although the ammonia-nitrogen (NH4+-N) concentration in the 0–20 cm layer was lower. There were significantly positive correlations between root biomass density and both soil nutrients and microbial biomass, while there was a negative correlation between the soil NH4+-N and root biomass density. These results indicate that alfalfa root growth improved soil organic matter accumulation and nutrient mineralization. The accumulation and mineralization of soil nutrients also guaranteed root and microorganism growth. Therefore, it was concluded that alfalfa introduction will promote soil nutrients immobilization and mineralization and may enable sustainable land use in the semi-arid region of the Loess Plateau, China.

Soil physical properties, infiltrability, and water flow behavior are closely associated with root morphology. In this study, dye tracer experiments were conducted in the field to assess the effects of roots with various morphometric features on soil water flow behavior and soil infiltrability. Preferential flow is the dominant type of water flow in the three study sites (Minqin, Dongtai, and Mengla). The presence of roots caused noncapillary porosity and saturated hydraulic conductivity to increase by 26% and 252% in the Minqin, 92% and 93% in the Dongtai, and 234% and 96% in the Mengla, respectively, relative to that in in soils without roots. The various morphometric features of roots affect water flow behavior in soil. The fibrous roots of maize induced water to flow and distribute evenly throughout the plough layer of the study site in Minqin. Furrows and ridges in this site exhibited different soil physical and hydrological properties. Furrows could store higher amounts of water than ridges, thereby increasing the likelihood of water absorption by plants. In Dongtai, ponding and surface runoff occurred when water infiltration was inhibited by the mud layer, which exhibited a high soil bulk density value of 1.43 g cm−3 and a low saturated hydraulic conductivity value of 1.67 × 10−5 cm s−1. These phenomena were not observed in plots the smooth roots of Spartina alterniflora penetrated the mud layer. In this site, preferential flow and lateral flow, which is triggered by the sandy loam layer, are important for water discharge from the beach to the ocean. In Mengla, water flowed evenly on hard soil, which exhibited a high bulk density value of 1.43 g cm−3. The fine roots of rubber could guide water infiltration into deep soil layers (73.54 cm), thereby redistributing water to the root zones of rubber trees. Therefore, our findings indicated that water infiltration behavior, which is crucial for water distribution, is affected by the various morphometric features of roots.

Evaluating the responses of soil respiration and its components to global environmental change is crucial for predicting future terrestrial carbon cycle. However, the effects of warming and nitrogen (N) addition on them remain unclear. A field manipulative experiment was conducted in a semi-arid grassland on the Loess Plateau to evaluate the responses of soil total respiration (Rt), autotrophic respiration (Ra) and heterotrophic respiration (Rh) to warming and N addition from April 2015 to December 2016. Open-top chambers were used to elevate temperature and N was added as NH4NO3 at a rate of 4.42 g N m−2 yr−1. Warming significantly decreased Rt and Rh by 7.4% and 9.5%, respectively, but had no significant effect on Ra. N addition significantly stimulated Ra by 34%, whereas it inhibited Rh by 11% and had no significant effect on Rt. The maximum N-inducing stimulation of Rt and Ra were observed in the month with the most rainfall events in 2015. N addition significantly increased the contribution of Ra to Rt by 10%. Warming decreased the Q10 values of Rt and Rh but had no significant effect on Q10 values of Ra. N addition significantly increased the Q10 values of Rt and Rh, whereas it decreased the Q10 values of Ra. The combination of warming and N addition had a synergistic effect on the cumulant of Rh, whereas it had an antagonistic effect on Ra. No interactive effect between warming and N addition was observed on Rt. Our results emphasized that Ra and Rh responded differently to warming and/or N addition, and the extreme rainfall frequency influenced the responses of Rt and its components to N addition. Our findings suggested that Rt has the potential to resist climate warming and increasing N deposition by differentiating the responses of its inherent components.

Understanding the factors contributing to spatial variation in the functional structure of plant is crucial for revegetation and ecosystem management of the Loess Plateau, China. As one of the measures in the “Grain for Green” project, lucerne ( Medicago sativa L.) has been widely planted in this seriously-eroded region to improve revegetation and conserve soil and water. However, information on how environmental factors influence the long-term development of lucerne-rich vegetation is scarce. This study aimed to identify the main environmental variables controlling the lucerne-rich vegetation through multivariate analyses. Vegetation and soil surveys were performed in 28 fields containing 11-year-old lucerne. Vegetation variables were total aboveground biomass, aboveground biomass of lucerne, total cover, lucerne cover, total abundance, species richness, Shannon–Wiener’s diversity index, Simpson’s predominance index, and evenness index. Topographic variables were slope, slope position, and slope aspect. Soil variables were soil total nitrogen (N) and phosphorus (P), available P, mineral N, organic carbon (SOC), moisture content (SMC), microbial biomass C and N. Analyses verified that vegetation variables, soil variables, and topographic variables were significantly correlated. One of the most important factors that influence lucerne revegetation process was SOC. SMC, total P, slope, and mineral N were also key factors that influenced vegetation variables. The obtained values of total aboveground biomass and cover were still high after 11 years planting. The fields that in the north– and south–facing slopes did not significantly differ in terms of total cover, and total and lucerne aboveground biomass. The results suggest that soil properties (such as SOC, TN, and mineral N, etc.) and topographic variables (i.e., slope) interacted with each other and acted on plants simultaneously in the lucerne-rich vegetation. To revegetation the degraded ecosystems by lucerne planting, we should consider the effects of the changes of soil properties, such as SOC, SMC, and soil P, on vegetation development.

Different rubber-based agroforestry systems are adopted to control and mitigate land degradation. However, the soil physical conditions and soil hydrological processes of different agroforestry systems are still unclear. Thus, in this study, rubber (Hevea brasiliensis) monoculture, rubber and Clerodendranthus spicatus agroforestry system (RCS) and rubber and Amomum villosum agroforestry system (RAV) were developed from a degraded land that had similar backgrounds of terrain and management measures for 50 years. Conventional methods were applied, and dye tracer experiments were conducted to measure the soil physical conditions and determine the water movement in soil. After 5 years' growth, both RCS and RAV could effectively promote the soil physical conditions and optimize soil structure by improving the proportion of the three soil phases. Favourable soil properties, multiple-layered canopies and ground cover in agroforestry systems could promote the formation of three-dimensional hydraulic redistribution in soil profile. The infiltration of rainfall into the soil was enhanced; meanwhile, surface runoff and soil erosion were mitigated, and then more water was transported, redistributed and stored into the different soil layers by the more dominant preferential flow, water exchange and lateral flow in soil profiles. These water supply mechanisms could allow planting intercrops with rubber trees to uptake water from different water sources and coexist in an agroforestry system. Our results highlighted that rubber-based agroforestry systems are a useful management practice to maximize the utilization of land and water resources. Copyright © 2017 John Wiley & Sons, Ltd.

Lucerne (Medicago sativa L.) is widely planted on the semi-arid Loess Plateau in China to control soil erosion, yet the extent to which topography affects the productivity of lucerne still remains poorly understood. This study aimed to evaluate how topographic position influences soil properties and aboveground biomass in lucerne-rich vegetation. A total of 112 quadrats were established in 28 fields covered with 11-year-old lucerne vegetation. In each quadrat, the slope, slope position, slope aspect, altitude, soil properties (soil moisture content, organic carbon, total nitrogen [N] and phosphorus [P], available P and inorganic N), and aboveground biomass were measured. Redundancy and multiple regression analyses were performed to determine the relationships among topographic factors, soil properties and aboveground biomass. A modest proportion of variation in soil variables was explained by topographic variables. The altitude, slope and slope position, rather than the slope aspect, were the key factors that influencing soil variables. Soil organic carbon, total N, inorganic N, the ratio of organic carbon to available P (C/P), and the ratio of total N to available P were positively correlated with altitude, whereas available P was negatively correlated with altitude. The soil moisture content was primarily affected by the slope and slope position. The topographic factors did not directly affect the total and lucerne aboveground biomass. The total and lucerne aboveground biomass were positively correlated to soil moisture content, inorganic N and C/P. Therefore, the aboveground biomass of lucerne-rich vegetation could be indirectly regulated by the slope and slope position through the effect on the soil moisture content and by altitude through the effect on soil inorganic N and C/P. This work highlighted the importance of soil properties such as soil moisture and P dynamics in the revegetation process of lucerne in this semi-arid region. As soil properties can be directly governed by topography, considering topography could enhance the quality of vegetation restoration in the large hilly region of the Loess Plateau.

The accrual of soil organic carbon (SOC) and nitrogen (N) in grassland is an important management option to improve the ecosystem functions of grassland. However, how abiotic (such as grazing exclusion (GE)) and biotic factors influence SOC and N and their different fractions in Tibetan alpine meadows remains unclear. In this study, we evaluated the relative importance of abiotic and biotic factors that drive SOC and N contents in soil density fractions by performing redundancy analysis based on three long-term (10 years) fenced alpine meadows maintained in the permafrost region of the Tibetan Plateau in China. Biotic factors comprise plant aboveground biomass, cover and diversity, whereas abiotic factors include soil properties (i.e. soil moisture, pH, clay, silt, sand, total phosphorus, available phosphorus, microbial biomass carbon and N, available N, C:N ratio and C:P ratio) and GE. Site rather than GE has significant effects on the SOC and N contents. GE caused no increase in the SOC and N contents in the whole soil and fractions. Redundancy analysis showed that 96.7% of the variations in SOC and N fractions can be explained by the selected explanatory variables. Aboveground biomass, cover, soil moisture and clay contents were key factors that affected the SOC and N fractions. The SOC and N fractions were mainly explained by the interaction between vegetation and soil, followed by soil, vegetation and GE. The study highlighted the importance of considering the covariation of vegetation and soil for evaluating the SOC and N dynamics in alpine meadows. The effect of GE (such as 10 years) on the SOC and N contents in alpine meadows can be weak in the permafrost region of the Tibetan Plateau.

Revegetation of degraded arable land with legume pasture has been proposed as an effective approach to improve vegetation conditions and control soil erosion on the Loess Plateau, China. This paper investigated the effects of revegetation on soil carbon pool and its fractions. Three treatments (natural revegetation, Medicago sativa L. introduction and Melilotus suaveolens L. introduction) were applied over 11 years from 2003 to 2013 on the semi-arid Loess Plateau; both aboveground biomass and soil organic carbon (SOC) stocks as well as its fractions (i.e., the light-fraction organic carbon (LFOC) and heavy-fraction organic carbon (HFOC)) were measured. We found that plant aboveground biomass was greater under Melilotus treatment than under Medicago treatment and natural revegetation in the first two years. Afterward, the aboveground biomass was greatest under Medicago treatment. Medicago treatment yielded higher SOC and HFOC stocks than the other two treatments during the experimental period. After revegetation for 10 years, the SOC stocks at the 0–1.5 m soil layer under Medicago treatment were 52.8 and 44.8 Mg ha−1 higher than those under natural revegetation and Melilotus treatments, respectively. The mean rates of SOC and HFOC sequestration in the top 20 cm of the soil layer under Medicago treatment were 0.27 and 0.23 Mg ha−1 year−1 in year 11, which were greater than the other two treatments (0.22, 0.15 and −0.07, −0.03 under the two treatments, respectively). Revegetation through introduction of Medicago to degraded arable land can potentially promote SOC accumulation by enhancing the transformation from LFOC to HFOC on the semi-arid Loess Plateau.

Revegetation facilitated by legume species introduction has been used for soil erosion control on the Loess Plateau, China. However, it is still unclear how vegetation and soil resources develop during this restoration process, especially over the longer term. In this study, we investigated the changes of plant aboveground biomass, vegetation cover, species richness and density of all individuals, and soil total nitrogen, mineral nitrogen, total phosphorus and available phosphorus over 11 years from 2003 to 2013 in three treatments (natural revegetation, Medicago sativa L. introduction and Melilotus suaveolens L. introduction) on the semi-arid Loess Plateau. Medicago significantly increased aboveground biomass and vegetation cover, and soil total nitrogen and mineral nitrogen contents. The Medicago treatment had lower species richness and density of all individuals, lower soil moisture in the deep soil (i.e., 1.4-5m), and lower soil available phosphorus. Melilotus introduction significantly increased aboveground biomass in only the first two years, and it was not an effective approach to improve vegetation biomass and cover, and soil nutrients, especially in later stages of revegetation. Overall, our study suggests that M. sativa can be the preferred plant species for revegetation of degraded ecosystems on the Loess Plateau, although phosphorus fertilizer should be applied for the sustainability of the revegetation.

Grazing exclusion (GE) is widely used in the Tibetan Plateau of China to restore degraded grasslands. However, the effects of GE on vegetation and soil properties in alpine meadows are still not well understood. We compared GE with adjacent continuously grazing alpine meadows at five sites to evaluate the impact of GE (10–14 years) on vegetation and soil properties in the permafrost region of the Tibetan Plateau. We collected soil samples at two soil depths (0–20 and 20–40 cm) from GE and adjacent grazing sites. The results showed that GE and Site had significant effects on aboveground biomass and cover, Site but not GE had significant effects on diversity index. GE significantly increased the aboveground biomass and cover in three of the five sites, particularly where vegetation cover was greatest. At each site, GE had inconsistent effects on plant species diversity (a positive effect at one site, no significant effects at three sites, and a negative effect at the other site). No significant differences were found between GE and adjacent grazing grasslands in soil properties (soil organic carbon [SOC], total nitrogen [N], total phosphorus [P], available P, mineral N, microbial biomass carbon [MBC] and nitrogen [MBN] and pH) for both soil depths at each site. A general linear mixed model confirmed that site and sampling depth, rather than GE, significantly affected soil properties (except for MBC). Both total aboveground biomass and cover were positively correlated with soil total P, MBC and MBN, were negatively correlated with available P and pH, and were not significantly correlated with SOC and total N. The results indicate that long-term GE (10–14 years) in alpine meadow did not significantly impact soil properties, and its impact on plant biomass and vegetation cover depends on the vegetation condition of the site. Our results can provide new insights for alpine meadow management in the permafrost region of the Tibetan Plateau.

Responses of soil respiration (Rs) to increasing nitrogen (N) deposition and warming will have far-reaching influences on global carbon (C) cycling. However, the seasonal (growing and non-growing seasons) difference of Rs responses to warming and N deposition has rarely been investigated. We conducted a field manipulative experiment in a semi-arid alfalfa-pasture of northwest China to evaluate the response of Rs to nitrogen addition and warming from March 2014 to March 2016. Open-top chambers were used to elevate temperature and N was enriched at a rate of 4.42g m-2yr-1 with NH4NO3. Results showed that (1) N addition increased Rs by 14% over the two-year period; and (2) warming stimulated Rs by 15% in the non-growing season, while inhibited it by 5% in the growing season, which can be explained by decreased plant coverage and soil water. The main effect of N addition did not change with time, but that of warming changed with time, with the stronger inhibition observed in the dry year. When N addition and warming were combined, an antagonistic effect was observed in the growing season, whereas a synergism was observed in the non-growing season. Overall, warming and N addition did not affect the Q10 values over the two-year period, but these treatments significantly increased the Q10 values in the growing season compared with the control treatment. In comparison, combined warming and nitrogen addition significantly reduced the Q10 values compared with the single factor treatment. These results suggest that the negative indirect effect of warming-induced water stress overrides the positive direct effect of warming on Rs. Our results also imply the necessity of considering the different Rs responses in the growing and non-growing seasons to climate change to accurately evaluate the carbon cycle in the arid and semi-arid regions.