Large-scale soil organic carbon mapping based on multivariate modelling: The case of grasslands on the Loess Plateau

The Loess Plateau is considered one of the world's regions with severe soil erosion. Grasslands are widely distributed on the Loess Plateau, accounting for approximately 40% of the total area. Soil organic carbon (SOC) plays an important role in the terrestrial carbon cycle in this region. We compiled more than 1,000 measurements of plant biomass and SOC content derived from 223 field studies of grasslands on the Loess Plateau. Combined with meteorological factors (precipitation and air temperature) and the photosynthetically active radiation factor, the topsoil SOC contents of grasslands were predicted using the random forest (RF) regression algorithm. Predicted grassland SOC content (1.70-40.34gkg(-1)) decreased from the southeast to the northwest of the Loess Plateau, with approximately 1/5 of the grassland exhibiting values lower than 4gkg(-1). Observed SOC content was positively correlated with observed plant biomass, and for predicted values, this correlation was strong in the desert steppe and the steppe desert of rocky mountains. Air temperature was the most important factor affecting SOC contents in the RF model. Moreover, the residual error of observations and predictions increased as the grazing intensity varied from none to very severe in the temperate desert steppe, and this RF model may not perform well in plains. The use of the RF model for SOC prediction in Loess Plateau grasslands provides a reference for C storage studies in arid and semi-arid regions, and aboveground biomass and temperature should receive more attention due to increasing C sequestration

Estimates of carbon storage in grassland ecosystems on the Loess Plateau

Grassland ecosystems play an important role in the carbon (C) balance of arid and semi-arid regions. These ecosystems provide C for grass growth and soil microbial activities and represent one of the main sources of atmospheric C. In this study, we estimated the C density and storage of 223 sampling sites in grassland ecosystems on the Loess Plateau using elevation, vegetation indexes, precipitation, air temperature, day and night land surface temperature (LSTd and LSTn, respectively), evapotranspiration (ET), percent tree cover and the non-vegetated area to build decision regression tree and generalized linear regression models (GLMs). The results showed that the C density decreased from south to north and ranged from 0.22 to 29.29 kg C/m(2). The average amount of C stored in the ecosystems was 1.46 Pg. The typical steppe and forest steppe stored the most C, and the steppe desert stored the least. The soil (0-1 m) stored most of the organic C, accounting for > 90%, and the belowground biomass (BGB) contained > 3 times the amount of C as the aboveground biomass (AGB). This study provides reference information for the loss of C and associated mitigation strategies on the Loess Plateau

Leaf photosynthetic function duration during yield formation of large-spike wheat in rainfed cropping systems

Improving photosynthetic capacity significantly affects the yield of wheat (Triticum aestivum L.) in rainfed regions. In this study, the physiological characteristics of eight large-spike wheat lines were compared with a multiple-spike cultivar as a control (CK) in a field over two consecutive seasons: 2010–2012. The tillering peak was 7–21 d after returning green for line 2040, the average rate of decline of relative water content was slower, and the average duration time of photosynthetic rate was longer than CK in vitro. There was a strong linear and positive correlation between photosynthetic rate and root activity at jointing, flowering, and grain-filling stages. In addition, average yields were higher in large-spike lines than CK (multiple-spike cultivar). The results suggest that large-spike lines might have greater water retaining capacity during yield formation under rainfed conditions

Carbon in Chinese grasslands : meta-analysis and theory of grazing effects

Unidad de excelencia María de Maeztu CEX2019-000940-MGlobally, livestock grazing is an important management factor influencing soil degradation, soil health and carbon (C) stocks of grassland ecosystems. However, the effects of grassland types, grazing intensity and grazing duration on C stocks are unclear across large geographic scales. To provide a more comprehensive assessment of how grazing drives ecosystem C stocks in grasslands, we compiled and analyzed data from 306 studies featuring four grassland types across China: desert steppes, typical steppes, meadow steppes and alpine steppes. Light grazing was the best management practice for desert steppes (< 2 sheep ha−1) and typical steppes (3 to 4 sheep ha−1), whereas medium grazing pressure was optimal for meadow steppes (5 to 6 sheep ha−1) and alpine steppes (7 to 8 sheep ha−1) leading to the highest ecosystem C stocks under grazing. Plant biomass (desert steppes) and soil C stocks (meadow steppes) increased under light or medium grazing, confirming the 'intermediate disturbance hypothesis'. Heavy grazing decreased all C stocks regardless of grassland ecosystem types, approximately 1.4 Mg ha−1 per year for the whole ecosystem. The regrowth and regeneration of grasslands in response to grazing intensity (i.e., grazing optimization) depended on grassland types and grazing duration. In conclusion, grassland grazing is a double-edged sword. On the one hand, proper management (light or medium grazing) can maintain and even increase C stocks above- and belowground, and increase the harvested livestock products from grasslands. On the other hand, human-induced overgrazing can lead to rapid degradation of vegetation and soils, resulting in significant carbon loss and requiring long-term recovery. Grazing regimes (i.e., intensity and duration applied) must consider specific grassland characteristics to ensure stable productivity rates and optimal impacts on ecosystem C stocks

Spatial patterns of photosynthetic characteristics and leaf physical traits of plants in the Loess Plateau of China

The spatial patterns of photosynthetic characteristics and leaf physical traits of 171 plants belonging to nine life-forms or functional groups (trees, shrubs, herbs, evergreen trees, deciduous trees, C-3 and C-4 herbaceous plants, leguminous and non-leguminous species) and their relationships with environmental factors in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi and Shenmu, ranging from south to north in the Loess Plateau of China were studied. The results showed that the leaf light-saturated photosynthetic rate (P-max), photosynthetic nitrogen use efficiency (PNUE), chlorophyll content (Chl), and leaf mass per area (LMA) of all the plants in the Loess Plateau varied significantly among three life-form groups, i.e., trees, shrubs and herbs, and two groups, i.e., evergreen trees and deciduous trees, but leaf nitrogen content differed little among different life-form groups. For the 171 plants in the Loess Plateau, leaf P-max was positively correlated with PNUE. The leaf nitrogen content per unit area (N-area) was positively correlated but Chl was negatively correlated with the LMA. When controlling the LMA, the N-area was positively correlated with the Chl (partial r = 0.20, P &lt; 0.05). With regard to relationships between photosynthetic characteristics and leaf physical traits, the P-max was positively correlated with N (area), while the PNUE was positively correlated with the Chl and negatively correlated with the N-area and LMA. For all the species in the Loess Plateau, the PNUE was negatively correlated with the latitude and annual solar radiation (ASR), but positively correlated with the mean annual rainfall (MAR) and mean annual temperature (MAT). With regard to the leaf physical traits, the leaf Chl was negatively correlated with the latitude and ASR, but positively correlated with the MAR and MAT. However, the N-area and LMA were positively correlated with the latitude and ASR, but negatively correlated with the MAR and MAT. In general, leaf N-area and LMA increased, while PNUE and Chl decreased with increases in the latitude and ASR and decreases in MAR and MAT

Vertical distribution of fine roots in relation to soil factors in Pinus tabulaeformis Carr. forest of the Loess Plateau of China

Growth and vertical distribution of fine root closely depend on soil resource availability. Better understanding of relationships of root profile with vertical distribution of available soil resource and soil characteristics can allow ecologists to predict the fine root distribution on the scales ranging from individual plants to vegetation communities. The objective of the study was to understand the fine root mass density (FRMD), fine root length density (FRLD), fine root area density (FRAD), mean root diameter and specific root length (SRL), vertical distribution in soil profile and their relation with soil environment factors in semiarid and arid Loess Plateau of China. The vertical fine root distribution and soil bulk density, soil moisture and soil inorganic N in 0-60 cm soil profile (0-15, 15-30, 30-45 and 45-60 cm intervals) were investigated by soil coring methods in three Pinus tabulaeformis Carr. forests chosen at three locations. The fine root density parameters (FRMD, FRLD and FRAD) and SRL peaked in the most upper soil layer (0-15 cm interval) and decreased with increased soil depth. The results provided a strong support that soil water rather than soil inorganic N is a key control on fine root distribution in the Loess Plateau. With increased soil moisture, the root mass, length and SRL increased and the mean root diameter decreased. The effects of soil bulk density on the fine root parameters were consistent with those of the soil water. An unexpected result was obtained about the relationships between soil organic N and the root distributions and occurrences because of no differences among the soil depth intervals in soil inorganic N content. It might be associated with severe soil water deficit limiting soil nitrogen utilization efficiency in arid Loess Plateau

Spatial patterns of leaf nutrient traits of the plants in the Loess Plateau of China

The spatial patterns of leaf nutrient traits of plants in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi and Shenmu, standing from south to north in the Loess Plateau of China, were studied. The results showed that of the 126 plant samples in the Loess Plateau, the mean leaf organic carbon (C), nitrogen (N), phosphorus (P) and potassium (K) were 43.8, 2.41, 0.16 and 1.67%, respectively, and ranked in the order of C &gt; N &gt; K &gt; P. Leaf C, N, P and K ranged from 32.6 to 54.8%, 0.82 to 4.58%, 0.06 to 0.35%, and 0.24 to 4.21%, respectively. The mean leaf C/N, C/P and N/P ratios were 21.2, 312 and 15.4, respectively. It is indicated that leaf N in the Loess Plateau was significantly higher than those in Chinese and global flora, but leaf P was significantly lower than that in global flora, which resulted in a higher N/P ratio in the Loess Plateau. The results also showed that leaf C, N, P, K, C/N and C/P ratios varied significantly among the seven life-form groups, which were trees, shrubs, herbages, evergreen trees, deciduous trees, C-3 and C-4 herbages, but leaf N/P ratio differed little among the seven life-forms. In the sampled species in the Loess Plateau, leaf C was negatively correlated with leaf N, P and K, while leaf N, P and K were positively correlated with one another. In general, leaf N/P ratio increased as the latitude and annual solar radiation increased and the mean annual rainfall and mean annual temperature decreased

Runoff hydraulic characteristics and sediment generation in sloped grassplots under simulated rainfall conditions

Evaluation of grass influence on soil erosion process can provide important information in soil and water conservation. The laboratory experiment was conducted to study runoff and sediment producing processes and runoff hydraulics in the grassplots with different covers (35%, 45%, 65% and 90%) and bare soil plot (control) at a slope of 15. The results showed that grass significantly reduced runoff and sediment. Compared with bare soil plot, the grassplots had a 14-25% less runoff and an 81-95% less sediment, and played a more important role in reducing sediment at the final stage of rainfall. There was a significantly negative logarithmic relationship between sediment yield rate (SDR) and cover (C): SDR = 1.077-2.911 ln(C) (R-2 = 0.999'&quot;). Sediment yield rate of grassplots decreased with rainfall duration, and decreased linearly as runoff rate increased. Overland flow velocities deceased with increase in grass cover, and the cover had greater effect on lower slope velocity than upper one. Froude numbers decreased with increase in cover, and flow regimes of all treatments were laminar and tranquil. Darcy-Weisbach and Manning friction coefficients of grassplots increased as ground cover increased. Therefore, increase in grass coverage can efficiently reduce soil loss and improve ecological environments. (c) 2006 Elsevier B.V. All rights reserved

Water consumption characteristics and water use efficiency of winter wheat under long-term nitrogen fertilization regimes in northwest China.

Water shortage and nitrogen (N) deficiency are the key factors limiting agricultural production in arid and semi-arid regions, and increasing agricultural productivity under rain-fed conditions often requires N management strategies. A field experiment on winter wheat (Triticum aestivum L.) was begun in 2004 to investigate effects of long-term N fertilization in the traditional pattern used for wheat in China. Using data collected over three consecutive years, commencing five years after the experiment began, the effects of N fertilization on wheat yield, evapotranspiration (ET) and water use efficiency (WUE, i.e. the ratio of grain yield to total ET in the crop growing season) were examined. In 2010, 2011 and 2012, N increased the yield of wheat cultivar Zhengmai No. 9023 by up to 61.1, 117.9 and 34.7%, respectively, and correspondingly in cultivar Changhan No. 58 by 58.4, 100.8 and 51.7%. N-applied treatments increased water consumption in different layers of 0-200 cm of soil and thus ET was significantly higher in N-applied than in non-N treatments. WUE was in the range of 1.0-2.09 kg/m3 for 2010, 2011 and 2012. N fertilization significantly increased WUE in 2010 and 2011, but not in 2012. The results indicated the following: (1) in this dryland farming system, increased N fertilization could raise wheat yield, and the drought-tolerant Changhan No. 58 showed a yield advantage in drought environments with high N fertilizer rates; (2) N application affected water consumption in different soil layers, and promoted wheat absorbing deeper soil water and so increased utilization of soil water; and (3) comprehensive consideration of yield and WUE of wheat indicated that the N rate of 270 kg/ha for Changhan No. 58 was better to avoid the risk of reduced production reduction due to lack of precipitation; however, under conditions of better soil moisture, the N rate of 180 kg/ha was more economic

Influences of grass and moss on runoff and sediment yield on sloped loess surfaces under simulated rainfall

&nbsp;It is important to evaluate the impacts of grasses on soil erosion process so as to use them effectively to control soil and water losses on the Loess Plateau. Laboratory-simulated rainfall experiments were conducted to investigate the runoff and sediment processes on sloped loess surfaces with and without the aboveground parts of grasses and moss (GAM: grass and moss; NGAM: no grass and moss) under slope gradients of 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees and 30 degrees. The results show that runoff from GAM and NGAM plots increased up to a slope gradient of 10 degrees and decreased thereafter, whereas the runoff coefficients increased with gradient. The average runoff rates and runoff coefficients of NGAM plots were less than those of GAM plots except for the 5 degrees slope. This behaviour may be due to the reduction in water infiltration under moss. The difference between GAM and NGAM plots in average runoff rates varied from 1.4 to 8%. At the same gradients, NGAM plots yielded significantly (alpha = 0.05) more sediment than GAM plots. Average sediment deliveries for different slopes varied from 0.119 to 3.794 g m(-2) min(-1) from GAM plots, and from 0.765 to 16.128 g m(-2) min(-1) from NGAM plots. Sediment yields from GAM plots were reduced by 45 to 85%, compared with those from the NGAM plots. Plots at 30 degrees yielded significantly higher sediments than at the other gradients. Total sediments S increased with slope gradients G in a linear form, i.e. S = 9.25G - 39.6 with R-2 = 0.77*, for the GAM plots, and in an exponential model, i.e. S = 40.4 exp(0.1042G) with R-2 = 0.93**, for the NGAM plots. In all cases, sediment deliveries decreased with time, and reached a relative steady state at a rainfall duration of 14 min. Compared with NGAM plots, the final percentage reductions in sediment delivery from GAM plots were higher than those at the initial time of rainfall at all slopes. Copyright (c) 2006 John Wiley &amp; Sons, Ltd