Mineral N stock and nitrate accumulation in the 50 to 200 m profile on the Loess Plateau

Nitrogen (N) stored in deep profiles is important in assessing regional and/or global N stocks and nitrate leaching risk to groundwater. The Chinese Loess Plateau, which is characterized by significantly thick loess deposits, potentially stores immense stocks of mineral N, posing future threats to groundwater quality. In order to determine the vertical distributions of nitrate and ammonium content in the region, as well as to characterize the potential accumulation of nitrate in the deep loess profile, we study loess samples collected at five sites (Yangling, Changwu, Fuxian, An'sai and Shenmu) through a 50 to 200 m loess profile. The estimated storage of mineral N varied significantly among the five sites, ranging from 0.46 to 2.43 × 104 kg N ha−1. Ammonium exhibited fluctuations and dominated mineral N stocks within the whole profile at the sites, except for the upper 20–30 m at Yangling and Changwu. Measured nitrate content in the entire profile at Fuxian, An'sai and Shenmu is low, but significant accumulations were observed to 30–50 m depth at the other two sites. Analysis of δ15N and δ18O of nitrate indicates different causes for accumulated nitrate at these two sites. Mineralization and nitrification of manure and organic N respectively contribute nitrate to the 0–12 and 12–30 m profile at Changwu; while nitrification of NH4+ fertilizer, NO3− fertilizer and nitrification of organic N control the nitrate distribution in the 0–3, 3–7 and 7–10 m layer at Yangling, respectively. Furthermore, our analysis illustrates the low denitrification potential in the lower part of the vadose zone. The accumulated nitrate introduced by human activities is thus mainly distributed in the upper vadose zone (above 30 m), indicating, currently, a low nitrate leaching risk to groundwater due to a high storage capacity of the thick vadose zone in the region

Rapid soil water recovery after conversion of introduced peashrub and alfalfa to natural grassland on northern China’s Loess Plateau

To evaluate the potential of soil water recovery after thinning, in situ soil water content in the 0–500 cm soil profile under thinned (50%–100%) and unthinned peashrub and alfalfa plots and a nearby natural grassland in the Liudaogou watershed in China’s Loess Plateau (CLP) was measured monthly during 2015–2017 growing season using a neutron probe. At the start of experiment, the profile soil water storage (SWS0–500 cm) under introduced peashrub and alfalfa was, respectively, 18.8% and 12.2% lower than that under natural grassland. This showed that there was higher water consumption by planted vegetation, compared with native grass. After thinning, SWS0–500 cm in thinned peashrub and alfalfa plots was significantly higher than that in unthinned plots due to decrease in both interception and transpiration. The increase in SWS0–500 cm in the 100% thinned peashrub plot (159.9–216.1 mm) was much higher than that in 50% thinned peashrub (39.1–169.8 mm) and 100% thinned alfalfa (20.3–118.1 mm) plots. This indicated that the extent of soil water recovery varied with thinning intensity and vegetation type. At the end of the third growing season, soil water restoration frontier in the thinned peashrub and alfalfa plots (>300 cm) was much greater than that in the unthinned plots (<180 cm). It also indicated that with thinning, soil water (<300 cm) can recover rapidly following two successive wet years. The results suggested that concerns about soil desiccation and the potential impact on long-term sustainability of restored ecosystems on CLP were resolvable.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Factors that Influence the Vertical Distribution of Soil Water Content in the Critical Zone on the Loess Plateau, China

In arid and semiarid regions, determining the vertical distribution of the soil water content (SWC) in the Earth’s Critical Zone is important for understanding hydrological processes and for evaluating soil water storage (SWS) levels. However, the vertical distribution of SWC and its storage in deeper layers are unclear due to the difficulties associated with soil sampling. In this study, we investigated the vertical distribution of the SWC and SWS, and analyzed the relationship between SWC and related soil properties, including bulk density (BD), sand, silt, clay, and soil organic C (SOC), from the top of the soil profile to the bedrock at five sampling sites on the Loess Plateau in China (Yangling, Changwu, Fuxian, An’sai, and Shenmu) by soil core drilling. The results showed that the SWC variations at the five sampling sites tended to become weak as the depth increased. The mean SWC of all sampling sites exhibited a decreasing trend from south to north, with a significant difference ( < 0.01). Stepwise multiple regression analysis and state-space modeling showed that the BD was an important factor that affected the variations in the SWC in the deep soil layer. The trend of vertical distribution of the SWS was similar to that of the SWC. The results of this study deepen our understanding of the water conditions in deep soil layers, as well as the evaluation of SWS on the Loess Plateau in China

Spatial variability of soil carbon and water storage across loess deposit catenas in China’s Loess Plateau region

The impact of hillslope vegetation restoration on the distribution and variability of carbon and water storage was studied across two catenary sequences of soils in the Liudaogou watershed of China’s Loess Plateau. Soil organic carbon storage (SOCS) under different land uses in the two catenas decreased significantly in the upper soil layers (50 cm). However, soil inorganic carbon storage (SICS) in the two catenas fluctuated (two maxima) with increasing soil depth. There was no significant difference of SOCS within the 200 cm soil profile between forestlands (FO) and grasslands (GR) at the catenary scale (p > 0.05). However, SICS in the 0–200 cm soil profile differed markedly between FO and GR (p < 0.05) in both catenas due to different degrees of root-facilitated CaCO3 redistribution. Based on the coefficient of variance (CV), soil water storage (SWS) was divided into three layers: active layer (0–100 cm, CV = 20%–30%), subactive layer (100–200 cm, CV = 10%–20%), and stable layer (200–500 cm, CV < 10%). The SWS in the 0–500 cm soil profile was slightly higher in GR than in FO on the two slopes because of the higher water consumption under tree plantation than native grasses. SOCS, SICS, and SWS can be predicted by multiple regression equations using different soil properties. The study demonstrated that SOCS, SICS, and SWS respond differently to vegetation restoration at the catenary scale, which must be taken into account for improving ecosystem model predictions of soil carbon and water fluxes in sloping lands.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Re-evaluation of organic carbon pool from land surface down to bedrock on China's Loess Plateau

Large reservoirs of organic carbon (OC) store in deep soils ( > 1 m below land surface) are not usually included in regional and global terrestrial C inventories. Chinaos Loess Plateau (CLP), which has the worldos deepest loess deposit and has experienced long-term, intensive agricultural and revegetation activities, could contain large stores of OC. In this study, the distribution of OC concentration and stock across the entire loess profile from the ground surface down to the bedrock (56-205 m) was assessed at five sites (Yangling, YL; Changwu, CW; Fuxian, FX; Anosai, AS; and Shenmu, SM) under three land use types (farmland, grassland and shrubland). There was pronounced decrease in mean OC concentration with increasing depth along loess profiles at all the investigated sites. OC concentration in the topmost 20 m of the loess was much higher and fluctuated more significantly than that in the deeper layers at YL, CW, FX and AS, where mean annual precipitation was > 550 mm. In contrast, OC concentration was low and stable at SM with mean annual precipitation < 450 mm. The restoration of vegetation with deep roots following the abandonment of farmlands resulted in deep OC accumulation at YL and AS, whereas natural grassland recovery did not result in the accumulation of OC at SM. Our results suggested that land use change could alter OC distribution in deep soils, of which degree depended on climatic condition and vegetation type. The estimated OC storage varied significantly across the sites (859-5044 Mg ha(-1)), which mainly depended on the thickness of loess deposit. OC store on the CLP and other areas around the world with deep soils or sediments could be underestimated and need re-consideration in future C budget studies. This is the first estimate of regional OC stock across loess profile, spanning from the land surface down to the bedrock. The findings could have significant implications for biogeochemical cycling of C in surface and deep soil layers down to the bedrock

The use of chronosequences in studies of paddy soil evolution: A review

Chronosequences and associated space-for-time substitutions are an important and fruitful means for investigating the rates and directions of soil and ecosystem evolution across multiple time-scales ranging from decades to millions of years. This paper reviews the use of chronosequences for studying biogeochemistry of paddy soil evolution to improve our understanding of the fundamental processes, the dynamic changes in soil properties and the associated environmental thresholds at different stages of paddy soil evolution under the intensive anthropogenic managements. Rice paddy cultivation results in accumulations of various nutrients (e.g. organic carbon, nitrogen, and phosphorus) over a much longer time period than predicted by typical long-term (b50 years) field experiments, although it is not clear how long it takes paddy soils with different origins to reach a steady-state of these important nutrients. Extensive investigations of a 2000-year paddy soil chronosequence derived from calcareous marine sediments in the coastal region of Zhejiang Province (P.R. China) illustrate three phases of paddy soil evolution and the associated pedogenic thresholds: an initial phase during the first few decades dominated by rapid desalinization, loss of magnetic susceptibility, accumulation of topsoil organic matter and formation of a compacted plow pan due to extrinsic thresholds resulting from anthropogenic activities; the second phase lasts several centuries comprising Fe and clay enrichment in the illuvial horizon, and the loss of phosphorus and Mn coincident with the near complete removal of CaCO3 (recognized as the intrinsic threshold); in the third phase (N700 years), (trans-)formation and redistribution of metal oxides are accompanied by clearly visible hydromorphic patterns in paddy subsoils. We also note that after 2000-years, paddy soils still lack evidence of silicate weathering and neo-formation of pedogenic clay minerals. Paddy soil management is adjusted to match landscape positions (e.g. well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained bog areas with near surface water table) and this influences the trajectory and magnitude of pedogenic changes with prolonged rice cultivation. However, the parent material effects on paddy soil evolution seem to diminish with the lapse of time and vary considerably among different soil properties or processes. Given our universal dependence on paddy soils for food production, their value as an excellent opportunity for investigating anthropedogenesis, and their critical roles in global biogeochemical cycling, we put forward several open questions that must be resolved to maintain the millennial-scale sustainability of these important wetlands.</p

Development of pedotransfer functions for soil hydraulic properties in the critical zone on the Loess Plateau, China

Soil hydraulic properties (SHPs) including the soil water retention curve and saturated soil hydraulic conductivity (Ks) are crucial input data for simulations of soil water and solute transport in the Earth's critical zone. However, obtaining direct measurements of SHPs at a wide range of scales is time consuming and expensive. Pedotransfer functions (PTFs) are employed as an alternative method for indirectly estimating these parameters based on readily measured soil properties. However, PTFs for SHPs for the deep soil layer in the Earth's critical zone are lacking. In this study, we developed new PTFs in the deep soil profile for Ks and soil water retention curve on the Loess Plateau, China, which were fitted with the van Genuchten equation. In total, 206 data sets comprising the hydraulic and basic soil properties were obtained from three typical sites. Samples were collected from the top of the soil profile to the bedrock by soil core drilling. PTFs were developed between the SHPs and basic soil properties using stepwise multiple linear regression. The PTFs obtained the best predictions for Ks (R-adj(2)=0.561) and the worst for van Genuchten (R-adj(2)=0.474). The bulk density and sand content were important input variables for predicting Ks, , and s, and bulk density, clay content, and soil organic carbon were important for n. The PTFs developed in this study performed better than existing PTFs. This study contains the first set of PTFs of SHPs to be developed for the deep profile on the Loess Plateau, and they may be applicable to other regions

H2O2 induces PP2A demethylation to downregulate mTORC1 signaling in HEK293 cells

Mammalian target of rapamycin (mTOR) is a Ser/Thr protein kinase that functions as an ATP and amino acid sensor to govern cell growth and proliferation by mediating mitogen- and nutrient-dependent signal transduction. Protein phosphatase 2A (PP2A), a ubiquitously expressed serine/threonine phosphatase, negatively regulates mTOR signaling. Methylation of PP2A is catalyzed by leucine carboxyl methyltransferase-1 (LCMT1) and reversed by protein phosphatase methylesterase 1 (PME-1), which regulates PP2A activity and substrate specificity. However, whether PP2A methylation is related to mTOR signaling is still unknown. In this study, we examined the effect of PP2A methylation on mTOR signaling in HEK293 cells under oxidative stress. Our results show that oxidative stress induces PP2A demethylation and inhibits the mTORC1 signaling pathway. Next, we examined two strategies to block PP2A demethylation under oxidative stress. One strategy was to prevent PP2A demethylation using a PME-1 inhibitor; the other strategy was to activate PP2A methylation via overexpression of LCMT1. The results show that both the PME-1 inhibitor and LCMT1 overexpression prevent the mTORC1 signaling suppression induced by oxidative stress. Additionally, LCMT1 overexpression rescued cell viability and the mitochondrial membrane potential decrease in response to oxidative stress. These results demonstrate that H O induces PP2A demethylation to downregulate mTORC1 signaling. These findings provide a novel mechanism for the regulation of PP2A demethylation and mTORC1 signaling under oxidative stress