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To investigate the effects of shrubification—the global phenomena of an increase of shrubs in grasslands—on C, N, and P cycles, the changes in soil organic C, N, and P mineralization in an alpine meadow were compared across five plant communities: grasses and four shrub species–dominated patches. The nutrient content and stoichiometry (C:N:P) of leaves, litter, microbial biomass, and soil organic matter (SOM) were analyzed during vegetation season. Net rates of N and P mineralization were measured in situ in the top 20 cm of soil throughout the growing season, and organic C mineralization was determined under controlled conditions. Microbial C:N, C:P, and N:P ratios in the top 20 cm generally decreased with increasing plant size (height combined with crown diameter), associated with greater input of litter with lower C:nutrient ratios under shrubs. The net N and P mineralization rates in soil under shrubs were about 3- to sevenfold and 4- to 15-fold faster, respectively, compared with those under grasses. The increase in organic C mineralization under shrubs compared with that under grasses was much smaller than the increase of N or P mineralization under shrubs. This indicates faster turnover of nutrients than C leading to decoupling of organic C and nutrient mineralization across plant communities by shrubification. The C:N, C:P, and N:P ratios of organic pools mineralized in soil decreased with increasing plant size, but increased with respective microbial C:N, C:P, and N:P ratios across plant communities. This indicates that specific SOM pools were mineralized depending on plant communities and microbial stoichiometry in soil. Consequently, the decoupling of organic C and nutrient mineralization across plant communities is driven by microbial stoichiometry and increases by shrubification. [ABSTRACT FROM AUTHOR]

Little is known about the effects of geomorphologic features on the in situ mineralization of soil nitrogen (N) in high-altitude region. We evaluated the effects of aspect–vegetation complex on the in situ (net and gross) mineralization of soil N in 2012 and 2013. Two hill sites were randomly selected, both with vegetation naturally divided by a ridge into south-facing meadows and north-facing shrublands. Despite lower total soil organic carbon and N contents, the gross and net rates of soil N mineralization were greater in meadow than in shrubland at both sites. Over the growing season, the total cumulative net N mineralization across both sites averaged 114% greater under the meadow (7.9 g m − 2 ) than under the shrubland (3.7 g m − 2 ) in the upper 15 cm soil layer, and 155% greater under the meadow (22.2 g m − 2 ) than under the shrubland (8.7 g m − 2 ) in the upper 30 cm soil layer. The faster gross and net soil N mineralization in the meadow was primarily ascribed to 2–9 °C higher daily soil temperature compared to the shrubland, measured at 7.5 cm soil depth from June to October 2013. Despite the slower mineralization of soil N in the shrubland than in the meadow, there was an abundant inorganic N pool in the topsoil under the former compared to the latter. This was possibly linked to a lower uptake of N by the plants as a result of the slower growth of shrubs than herbaceous grasses. We concluded that the aspect–vegetation complex profoundly affected the cycling of soil N in mountainous areas of the Tibetan Plateau, highlighting the complexity in predicting soil N dynamics in future climatic and land management scenarios. [ABSTRACT FROM AUTHOR]

Plastic-film mulch is widely used to increase soil temperature and reduce water evaporation in vegetable production. In China, it is also extensively used for growing grain crops, especially in temperature and rainfall limited areas. However, it remains unclear whether the technology is sustainable in terms of maintenance of soil organic carbon (SOC) balance. We assessed the effects of plastic-film mulch on the SOC balance in maize ( Zea mays L.) production in a range of cold semiarid environments. We imposed four treatments: (i) no plastic-film mulch or straw incorporation, (ii) plastic-film mulch, (iii) straw incorporation in soil without mulch, and (v) straw incorporation plus mulch, in ridge–furrow prepared fields at five sites along a hydrothermal gradient for up to six years. Maize root biomass across sites increased by 23–38% in mulched plots associated with the increase in aboveground biomass, indicating an increased SOC input, compared to that in non-mulched plots. The plastic-film mulch increased SOC mineralization, indicated by the stimulated decomposition of buried maize straw, and a 4–5% reduction in the concentration of light-fraction SOC (<1.8 g cm −3 ), but the total SOC concentration and stock in the 0–0.15 m soil layer did not change relative to no mulch after six years of continuous cropping. Plastic-film mulch did not affect the total non-cellulosic sugar content; however, it significantly increased the contribution of microbial-synthesized sugars to the total non-cellulosic sugars, indicating an intensified microbial action on the SOC pool compared to no mulch. Straw incorporation increased the root biomass, light and total SOC concentrations and non-cellulosic sugars, and changed the non-cellulosic sugar composition. We conclude that the increase in soil temperature and moisture by use of plastic-film mulch enhances productivity, but importantly maintains the SOC level in temperature- and rainfall-limited semiarid regions by balancing the increased SOC mineralization with increased root-derived C input. [ABSTRACT FROM AUTHOR]

Abstract: Grazing exclusion has been proposed as a choice for restoring degraded grasslands on the Qinghai–Tibetan Plateau, but its effects on soil properties are not clear. The present study was designed to investigate whether various soil organic carbon (OC) and nitrogen (N) pools and enzymatic activities were changed through grazing exclusion. A paddock of grassland was fenced in May 2002 for exclusion of livestock grazing, while the surrounding grassland continued conventional grazing by yak (Bos grunniens) and sheep (Ovis aries). Eight years after grazing exclusion, besides a reduction in plant species, the root biomass and soil bulk density in the top 15-cm depth were reduced by 34% and 26%, respectively, compared to the grazed grassland. Grazing exclusion enhanced the C/N ratios of shoots and roots by 18–19%, indicating a quality reduction in the shoot and root litters compared with the non-exclusion. Grazing exclusion also lowered stocks of total soil OC and N, microbial biomass C and N, and acid-extracted carbohydrate C and soil enzymatic activities (per area) of β-glucosidase, urase, and phosphatase in the 0–15cm soil layer. Under grazing exclusion, less C input from the root-associated sources and possibly greater C output through heterotrophic respiration might have reduced various soil OC storages. However, a significant increase in soil mineral N pool was found under no grazing compared to grazing, possibly due to less plant N demand and uptake and change in N mineralization and/or immobilization. In conclusion, grazing exclusion is not beneficial to soil OC sequestration on the northeastern Qinghai–Tibetan Plateau. [Copyright &y& Elsevier]

The huge soil organic C (SOC) storage (around 34 Pg in the top 0.7 m) in Qinghai–Tibet Plateau (QTP) grasslands is commonly explained by slow decomposition of litter under cold climate therein, but this view may not be reliable as humidity also affects microbial activity. We sampled the 20 cm topsoil of grasslands along an altitudinal gradient from 1286 m on the western Loess Plateau (LP) to 4200 m above sea level on the northeastern QTP. The light-fraction SOC (LFOC), composition of non-cellulosic neutral carbohydrates, and amino sugars were used as biomarkers to investigate the intensity of microbial action on SOC as a function of climate along this altitudinal gradient. From the lowest-to the highest-humidity site with rising altitude, the root biomass tripled and the SOC content increased approximately sevenfold (from 13.5 g kg−1 to 93.3 g kg−1). The non-cellulosic neutral carbohydrate, microbial biomass C (MBC), and microbial necromass C (MNC) contents increased, but the LFOC content decreased. The contribution of MNC to the SOC and ratios between microbially- and plant-derived sugars in the non-cellulosic carbohydrate pool increased, but the proportion of LFOC in the SOC dropped. Consequently, besides the increased root biomass, the selective preservation of microbial compounds at colder and more humid sites contributed to SOC accruals in grasslands. The higher MBC in cold and humid grasslands perfectly explained the increased selective preservation of microbial derived C at the expense of plant C in higher-relative to lower-altitude areas. Importantly, the above humidity-controlled accumulations of microbial substances and SOC in grasslands were confirmed by results synthesized from published data across the LP and QTP. The higher SOC contents in cold and humid QTP grasslands relative to warm and dry regions were ascribed to the increased accumulation of microbial residues because of the increased humidity in QTP grasslands. • Soil organic C content increases with rising humidity from 1286 to 4200 m. a. s. l. • Proportion of light-fraction organic C in soil organic C decreases with increasing humidity. • Ratios of hexoses to pentoses in soil carbohydrates increases with rising humidity. • Proportion of microbial derived C in soil organic C increases with rising humidity. • Humidity increases preservation of microbial derived C in cold grasslands. [ABSTRACT FROM AUTHOR]

• Shrubs increase SOC content but decrease bulk density relative to grasses. • Shrubs doesn't increase SOC stock in the top 0.4 m soil relative to grasses. • Shrubs increase SOC stock in the lower profile compared with grasses. • Shallow sampling is insufficient for accurate estimation of SOC storage in shrubby meadows. The distribution of shrubs is expanding in grasslands of the Qinghai–Tibetan Plateau, but research is rare on the effects of this expansion on the storage of soil organic carbon (SOC). We explored the spatial heterogeneity of SOC by grid sampling in the upper 1.2 m soil profile in a grazed alpine shrubby meadow, in response to patchy distribution of shrub species (leguminous Caragana brevifolia and non-leguminous Potentilla fruticose , Spiraea alpina and Salix oritrepha). The SOC content in the top 0.4 m increased with the size (height and crown diameter) of shrubs, but the soil bulk density decreased with the size of the shrubs. As a result, the SOC stock in the top 0.4 m soil was generally similar among shrub and grass patches across the meadow. The increase in the SOC stock in the entire 1.2 m profile by 35 % under non-leguminous shrubs P. fruticosa , S. alpina and S. oritrepha (39.8–40.5 kg m−2) compared with grasses (29.7 kg m−2) was mainly due to the SOC accumulation at the lower 0.4–1.2 m. The leguminous shrub C. brevifolia did not affect the SOC stock in the 1.2 m soil (32.3 kg m−2) compared with the grasses. Consistent patterns of changes in the natural δ13C signature between plants and SOC across different types of vegetation patches provided robust evidence that the heterogeneity of SOC across the field was a result of the patchy distribution of vegetation. We emphasize the importance of soil sampling depth in assessing the impact of shrubification on SOC storage. The influence depth of shrubification on SOC storage is at least 1.2 m in the profile in grazed Qinghai–Tibetan grasslands. [ABSTRACT FROM AUTHOR]

[Display omitted] • 17-years after planting legumes increased but non-legumes unchanged SOC content versus abandonment. • Symbiotic N fixation by legumes increased microbial biomass production. • Symbiotic N fixation potentially decreased the recycling of N in microbial necromass. • Legumes efficiently increased SOC by increasing net microbial necromass accumulation. The planting of nitrogen (N)-fixing leguminous plants is a common measure to restore degraded vegetation and sequester soil organic carbon (SOC) in globally vast arid areas where N deficiency is a limiting factor for plants. However, the effect of leguminous plants on microbial synthesized SOC accumulation remains a knowledge gap. We investigated the effects of planting shrubs (leguminous Caragana korshinskii and non leguminous Salix psammophila) in abandoned croplands on microbial substance accumulation in soil compared with natural succession (dominated by non leguminous semi-shrub Artemisia desertorum) in an arid area. Seventeen years after plantation, C. korshinskii increased SOC stock in the top 20 cm across canopy and gap areas by 34–80%, but S. psammophila did not affect SOC compared with abandonment (A. desertorum). This variation in the SOC stock across three vegetation types was not consistent with change patterns of aboveground litter accumulation and root biomass, but was consistent with higher living microbial biomass and non living microbial necromass contents in the soil and their proportions in the SOC under leguminous C. korshinskii than non leguminous A. desertorum and S. psammophila. These results indicated that the increased SOC under the leguminous relative to non leguminous species was mainly ascribed to the increased accumulation of microbial synthesized substances under the legume. Microbial necromass N represented 56–59% of total soil N under C. korshinskii , but this proportion was only 25–39% under non leguminous species, responding to much greater plant N input and soil N availability under the legume. Therefore, we suggest that symbiotic N fixation by leguminous species increased the production of microbial biomass and potentially decreased the reuse of N from microbial necromass compared with non leguminous species. As such, legumes can efficiently increase the net accumulation of microbial necromass carbon and thus the SOC content in dryland soils. [ABSTRACT FROM AUTHOR]

Abstract: There is little information regarding the sustainability of the high yield agroecosystem with double ridges and furrows mulched with plastic film in semi-arid areas. In this study, we explored the sustainability of this agroecosystem with different mulching time during two growing seasons over 2006 and 2007. Three treatments were designed: (i) plastic film mulching applied at sowing and film removed at harvest (CK); (ii) mulching applied 30 days before sowing and removed at harvest (M1); and (iii) mulching applied at sowing and film left on field after harvest and used continually for mulching in the second season (M2). Microbial biomass C (MBC) and ratio of MBC to soil organic C (SOC) (MBC/SOC) were higher in M1 and M2 than in CK in 2007 growing season. The reduction rates of the ratios of light fraction of organic C (LFOC) to SOC (LFOC/SOC) with sampling dates were 0.0020, 0.0047 and 0.0045 for CK, M1 and M2, respectively. A larger value means a faster reduction rate of LFOC/SOC with sampling dates, and implying farming system would face a higher unsustainable risk. MBC correlated negative significantly with LFOC (R = −0.939, P = 0.0001) and mineral N (R = −0.835, P = 0.0007) due to low soil C pool. Accordingly, film mulched ridge and furrow system would threaten the sustainability of soil ecosystem via MBC increase a semi-arid agroecosystem. The practices of mulching applied 30 d before sowing (M1) and use of plastic film once every two years (M2) lead to increased environmental risk for the farming system. [Copyright &y& Elsevier]

Shrubification is widespread in global grasslands. However, whether shrubification promotes the mineralization of soil organic carbon (SOC) remains uncertain. We compared sizes and compositions of soil non-cellulosic and amino sugar pools, soil stoichiometry, SOC decomposition and its temperature sensitivity between herbaceous and shrubby patches in alpine meadows on the Qinghai-Tibetan Plateau. The SOC content and soil stoichiometric C/P and N/P ratios in shrubby patches markedly increased with increasing plant size compared with herbaceous areas, indicating the increased P limitation to microorganisms relative to C and N with SOC accumulation under shrubs. The mass ratio of galactose plus mannose to arabinose plus xylose in the non-cellulosic carbohydrate pool of plant input was higher under shrubs than under grasses but was generally similar in soils between herbaceous and shrubby patches. This suggests the retarded microbial transformation of plant-derived carbohydrates under shrubs. Shrubs decreased accumulation of microbial necromass in SOC but increased the proportion of fungal origin in the microbial necromass. Lower total CO 2 efflux per unit mass of SOC at either 15 °C or 25 °C from shrubby than from herbaceous patches over the 76-day incubation was mainly associated with the increased soil C/P or N/P ratio under shrubs. Shrubs decreased temperature sensitivity of SOC decomposition compared with grasses only at 20–40 cm soil depth, where microbial-synthesized substances had greater dominance over organic matter than at 0–20 cm depth and were less abundant under shrubs relative to grasses. Our findings show that increased P limitation to microorganisms in shrubification resulted in the decreased SOC decomposability and indicate that microbial-synthesized substances determined the temperature sensitivity of SOC mineralization. Therefore, shrubification mitigates CO 2 emissions in grasslands by decreasing SOC mineralization and its temperature sensitivity in the context of global warming. • Soil C/P and N/P ratios markedly increase with increasing plant size. • Shrubs retard microbial transformation of plant residues compared with grasses. • Shrubs decrease microbial necromass accumulation in SOM compared with grasses. • Increasing P limitation by shrubification reduces SOM decomposition. • Smaller microbial necromass in SOM under shrubs lowers decomposition temperature sensitivity. [ABSTRACT FROM AUTHOR]

Patchy distribution of herbaceous species is common in degraded grasslands on the Qinghai-Tibetan Plateau, but the effects of such a mosaic pattern on soil biochemical properties have rarely been studied. We compared soil organic carbon (SOC) mineralization, nitrogen (N) availability, ammonia oxidation and plant N uptake between the following vegetation patch types (plant communities) in an alpine meadow: grass (dominated by Elymus nutans , Poa crymophila and Leymus secalinus), sedge (Kobresia capillifolia and Kobresia humilis), Polygonum viviparum (P. viviparum only), Artemisia smithii (A. smithii only) and Oxytropis kansuensis (O. kansuensis only). All patches had a diameter ranging from about tens of centimeters to several meters. SOC mineralization was faster in patches beneath grass or A. smithii than in those beneath sedge or O. kansuensis , but the faster SOC mineralization beneath grass and A. smithii patches did not result in a difference in SOC storage in the top 30 cm of soil compared with that in other patches. Greater litter production beneath grasses or belowground carbon input (largest root/shoot ratio) beneath A. smithii may have offset the negative effect of rapid SOC mineralization on SOC storage under grass and A. smithii patches. Productive grasses were associated with the highest level of N availability in the soil, consistent with their greater demands for N uptake. Sedge species and A. smithii inhibited nitrification by decreasing the abundance of ammonia oxidation bacteria in soil, in contrast to grasses. The inhibition of biological nitrification under A. smithii explained the finding that the soil beneath this vegetation patch type had the lowest nitrate content as a percentage of total inorganic N. However, the inhibition of nitrification beneath sedges did not induce a decrease in the nitrate content as a percentage of total inorganic N under sedge patches compared with that under grass patches. This suggests that sedges preferentially use ammonium instead of nitrate as their N source. It is concluded that patchy distribution of herbaceous species in alpine meadows created significant spatial heterogeneity in soil biological and biochemical properties at the field scale. Sampling in ecosystem studies clearly needs to take into account such heterogeneity of soil properties and processes in grasslands with patchily distributed vegetation. • Soil organic carbon mineralized faster in grass patches than in sedge patches. • Productive grass species had highest soil nitrogen availability among patch types. • Sedge species and Artemisia smithii exhibited biological nitrification inhibition. • Plant patchiness induced heterogeneity in soil biology and processes in meadows. [ABSTRACT FROM AUTHOR]