Conversion Of Cropland Research Articles

Impact of land-use change and soil organic carbon quality on microbial diversity in soils across Europe.

Land-use and their change have dramatic consequences for above-ground biodiversity, but their impact on soil microbial communities is poorly understood. In this study, soils from 19 European sites representing conversion of croplands to grasslands or forests and of grasslands to croplands or forests were characterized for microbial abundance and bacterial diversity. The abundance of Bacteria and Fungi but not Archaea responded to land-use change. Site was the major determinant of the soil bacterial community structure, explaining 32% of the variation in 16S rRNA gene diversity. While the quantity of soil organic carbon (SOC) only explained 5% of the variation, SOC when differentiated by its quality could explain 22%. This was similar to the impact of soil pH (21%) and higher than that of land-use type (15%). Croplands had the highest bacterial diversity. Converting croplands to grassland caused an increase of Verrucomicrobia; croplands to forest increased Rhizobiales but decreased Bacteroidetes and Nitrospirae; and grasslands to cropland increased Gemmatimonadetes but decreased Verrucomicrobia and Planctomycetes. Network analysis identified associations between particular SOC fractions and specific bacterial taxa. We conclude that land-use-related effects on soil microorganisms can be consistently observed across a continental scale.

Policy Implications from Multi‐scale Watershed Models of Biofuel Crop Adoption across the Corn Belt

AbstractThe implications and value of Soil and Water Assessment Tool (SWAT)‐based simulations of the productive potential and water quality impacts associated with switchgrass, Miscanthus, or corn stover removal biofuel cropping systems are discussed. Specifically, the three accompanying studies describe the water quality implications of adopting the three biofuel‐cropping systems via large‐scale conversion of cropland or targeting to marginal lands for three smaller watersheds located in the western or eastern Corn Belt, or across the Upper Mississippi and Ohio‐Tennessee River Basins. Other results such as climate change related impacts for two eastern Corn Belt watersheds are also discussed. These studies are supported by the CenUSA Bioenergy coordinated agricultural project funded by the USDA to develop a regional system for producing cellulosic biofuels. A description of the evolving federal policy related to cellulosic biofuel production and consumption is provided as are other potential drivers for encouraging the adoption of stover removal, switchgrass, and Miscanthus as perennial feedstocks. Findings from the SWAT studies and their implications for environmental and economic performance in their respective agroecosystems are discussed, and commonalities and divergences in results are identified. The potential for policy design to improve the performance of these systems based on the findings of these modeling studies, and continuing research needs and directions for improved policy design are discussed. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.

Assessment of Bioenergy Cropping Scenarios for the Boone River Watershed in North Central Iowa, United States

AbstractSeveral biofuel cropping scenarios were evaluated with an improved version of Soil and Water Assessment Tool (SWAT) as part of the CenUSA Bioenergy consortium for the Boone River Watershed (BRW), which drains about 2,370 km2 in north central Iowa. The adoption of corn stover removal, switchgrass, and/or Miscanthus biofuel cropping systems was simulated to assess the impact of cellulosic biofuel production on pollutant losses. The stover removal results indicate removal of 20 or 50% of corn stover in the BRW would have negligible effects on streamflow and relatively minor or negligible effects on sediment and nutrient losses, even on higher sloped cropland. Complete cropland conversion into switchgrass or Miscanthus, resulted in reductions of streamflow, sediment, nitrate, and other pollutants ranging between 23‐99%. The predicted nitrate reductions due to Miscanthus adoption were over two times greater compared to switchgrass, with the largest impacts occurring for tile‐drained cropland. Targeting of switchgrass or Miscanthus on cropland ≥2% slope or ≥7% slope revealed a disproportionate amount of sediment and sediment‐bound nutrient reductions could be obtained by protecting these relatively small areas of higher sloped cropland. Overall, the results indicate that all biofuel cropping systems could be effectively implemented in the BRW, with the most robust approach being corn stover removal adopted on tile‐drained cropland in combination with a perennial biofuel crop on higher sloped landscapes. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.

Rural Household Energy Use and Its Determinants in China: How Important Are Influences of Payment for Ecosystem Services vs. Other Factors?

Fuelwood is an essential environmental good for livelihood in rural China. Heavy reliance on fuelwood is associated with numerous negative externalities. Due to decades of rapid economic growth in China, we hypothesize that the increase in rural household income would reduce reliance on fuelwood. Using data from a survey of 481 households conducted in 2014, we examined the effects of two payments for ecosystem services programs, the Conversion of Cropland to Forest Program (CCFP) and the Ecological Welfare Forest Program (EWFP), on energy use, along with other factors. We found that the CCFP did not significantly affect fuel choice nor the quantity of fuelwood used, but households with more forestland in EWFP were less likely to adopt modern fuels and likely to use more fuelwood. Household per capita annual income was the main factor promoting adoption of modern fuels, while household size was the most important factor determining the quantity of fuelwood used per capita. Overall, the gradually increasing adoption of modern fuels has thus far not resulted in any significant abandonment of fuelwood. Fuelwood remains the dominant fuel across all income groups, suggesting that households in the study area are in the early stages of the energy transition.

Monitoring cropland transition and its impact on ecosystem services value in developed regions of China: A case study of Jiangsu Province

Cropland use is experiencing an intense transition in both depth and breadth with rapid development of the Chinese economy. Unwarranted land use transitions can affect the ecosystem services value (ESV) and ecosystem functions significantly. This paper reports the dynamic patterns of cropland transition (CLT) and its impact on ESV in Jiangsu Province based on land-use data from 1990, 2000, 2010, and 2013, crop yield data, empirical data related to ESV, some revisions adapted to the situation of Jiangsu, and Geo-information Tupu methods. Jiangsu has experienced rapid economic growth and urbanization, which can be seen as a microcosm of China's development. Results showed that the total area of cropland decreased markedly from 1990 to 2013. This loss of cropland took place mainly through the expansion of construction land and water area, accounting for 83.78% and 13.71%, respectively. The gain in cropland came mainly from construction land, water area, grassland, and woodland, accounting for 58.92%, 19.92%, 11.46%, and 10.22%. The transitions from 2000 to 2010 were much more significant than during 1990–2000 and 2010–2013. CLTs were accompanied by relatively concentrated distributions near towns and cities and the distributions in southern and central Jiangsu were more than in northern Jiangsu. Between 1990 and 2013, CLTs gave rise to a decrease of 6.78 million US$ ESV that was mostly caused by the transition from cropland to construction land and water area to cropland. Based on this information, some of the major implications for improving the land use policy and ecological protection policy in China were discussed. These include increased emphasis on land quality and ecological environment in balance between cropland addition and conversion, introducing ESV measures to evaluate the ecological effect of land-use planning, and establish an all-around ecological compensation mechanism.

Hydrologic response to future land use change in the Upper Mississippi River Basin by the end of 21st century

AbstractThis study demonstrates the spatial variation in hydrologic processes across the Upper Mississippi River Basin (UMRB) by the end of 21st century, by ingesting FOREcasting Scenarios (FORE‐SCE) of Land‐use Change projections into a physics‐based hydrologic model—Soil and Water Assessment Tool. The model is created for UMRB (440,000 km2), using the National Landcover Database of year 2001 and climate data of 1991–2010. Considering 1991–2010 as the baseline reference period, FORE‐SCE projections of year 2091 under three scenarios (A1B, A2, and B1 from the Intergovernmental Panel on Climate Change) are separately assimilated into the calibrated model, whereas climate input is kept the same as in the baseline. Modeling results suggest an increase of 0.5% and 3.5% in the average annual streamflow at the basin outlet (Grafton, Illinois) during 2081–2100, respectively, for A1B and A2, whereas for B1, streamflow would decrease by 1.5%. Under the “worst case” A2 scenario, 6% and 133% increase, respectively, in agricultural and urban areas with 30% depletion of forest and grassland would result into 70% increase in surface runoff, 20% decrease in soil moisture, and 4% decrease in evapotranspiration in certain parts of the basin. Conversion of cropland, forest, or grassland to perennial hay/pasture areas would lower surface runoff by 25% especially in the central region, whereas persistent forest cover in the northern region would cause up to 7% increase in evapotranspiration. The ecosystem in the lower half of UMRB is likely to become adverse, as dictated by a composite water–energy balance indicator. Future land use change extents and resultant hydrologic responses are found significantly different under A2, A1B, and B1 scenarios, which resonates the need for multi‐scenario ensemble assessments towards characterizing a probable future. The spatial variation of hydrologic processes as shown here helps to identify potential “hot spots,” giving ways to adopt more effective policy alternatives at regional level.

Converting highly productive arable cropland in Europe to grassland: \u2013a poor candidate for carbon sequestration

Sequestration of atmospheric CO2 as organic carbon by agricultural soils (SOC) is promoted as a climate change mitigation option. IPCC provides guidelines for determining carbon stocks and sequestration potential, incentivising policy changes towards management of farmland for carbon sequestration. However, the basis of the assumption that agricultural soils can sequester significant atmospheric CO2 has been questioned. We sought to determine the potential for conversion of arable cropland to grassland to sequester carbon in the short to medium term and potential limiting factors. There were no differences in SOC stocks in the top 30 cm between grassland up to 17 years old and arable cropland at 14 sites across the UK. However, SOC showed different distribution patterns, being concentrated in the top 10 cm under grassland. Soil microbial communities were significantly different between arable and grassland, with higher biomass and lesser dominance by bacteria in grassland soils. A land use conversion experiment showed these changes occurred within one year of land use change. Failure of grassland soils to accumulate SOC was attributed to reduced available soil nitrogen, resulting in low productivity. The implications of these results for carbon sequestration in soils as a climate change mitigation strategy are discussed.

Land use change effects on ecosystem carbon budget in the Sichuan Basin of Southwest China: Conversion of cropland to forest ecosystem

In the humid subtropics, conversion of cropland to forest has been recognized to influence carbon cycling (e.g., soil CO2 emissions) and the associated ecosystem carbon balance. A three-year field study was conducted in situ to quantitatively evaluate effects of land use change on carbon budget in a cropland (under winter wheat-summer maize rotation) comparison with the adjacent forest ecosystem. During the three-year experimental period, on average, soil heterotrophic respirations were 35.19mgC·m−2·h−1 for the cropland and 40.02mgC·m−2·h−1 for the adjacent forest ecosystem. The quantified net primary production (NPP) were 8724.78kgC·ha−1·year−1 for the cropland (3218.14kgC·ha−1 for winter wheat season and 5506.64kgC·ha−1 for summer maize season) and 6478.99kgC·ha−1·year−1 for the adjacent forest ecosystem. Thus, the average positive net ecosystem production (NEP) of 5139.33kgC·ha−1·year−1 and 2790.43kgC·ha−1·year−1 were gained in the cropland and the adjacent forest ecosystem, respectively. Nonetheless, if take into consideration of crop grain harvest (i.e., removal), the mean NEP was only 976.69kgC·ha−1·year−1 for cropland which were over three-fold lower than for the adjacent forest ecosystem. The practice of conversion of cropland (maize-wheat rotation system) to forest consequently resulted in an average annual net carbon sequestration of 1813.74kgC·ha−1·year−1 in the study. Therefore, our findings highlight that practices of conversion of subtropical cropland to forest commonly conducted in the last decades act as sinks of atmospheric CO2 in southwest China.

Cropland has higher soil carbon residence time than grassland in the subsurface layer on the Loess Plateau, China

Carbon (C) residence time is one of the key factors that determine the capacity of C storage and the potential C loss in soil, but it has not been well quantified. Assessing C residence time is crucial for an improved understanding of terrestrial C dynamics. To investigate the responses of C residence time to land use, soil samples were collected from millet cropland (MC) and enclosed grassland (EG) on the Loess Platea, China. The soil samples were incubated at 25°C for 182days. A Bayesian inverse analysis was applied to evaluate the residence times of different C fractions based on the information contained in the time-series data from laboratory incubation. Our results showed that soil organic carbon (SOC) mineralization rates and the amount of SOC mineralized significantly was increased after the conversion of cropland into grassland due to the increase in labile C fraction. At the end of the incubation study, 2.1%–6.8% of the initial SOC was released. Our modeling analysis indicated that the sizes and pool of active C in EG were larger than those in MC. The residence times of slow C pool at the 10–30cm layers were longer in MC (11.8–15.7 years) than those in EG (14.7–21.9 years). Our inverse analysis also directly reflected that the active C pool dominated C mineralization in the first 92days, while the slow C pool was the largest contributor after 92days. Conversion of cropland to grassland increases SOC decomposition, probably through increased active C pool resulted from higher litter input and no ploughing activities. The declined SOC in subsoil may be attributed to the short carbon residence times in grassland.

Dynamics of soil organic carbon in density fractions during post-agricultural succession over two lithology types, southwest China

Agricultural abandonment has been proposed as an effective way to enhance soil organic carbon (SOC) sequestration. Nevertheless, SOC sequestration in the long term is largely determined by whether the stable SOC fractions will increase. Here the dynamics of SOC fractions during post-agricultural succession were investigated in a karst region, southwest China using a space-for-time substitution approach. Cropland, grassland, shrubland and secondary forest were selected from areas underlain by dolomite and limestone, respectively. Density fractionation was used to separate bulk SOC into free light fraction (FLFC) and heavy fraction (HFC). FLFC contents were similar over dolomite and limestone, but bulk SOC and HFC contents were greater over limestone than over dolomite. FLFC content in the forest was greater than in the other vegetation types, but bulk SOC and HFC contents increased from the cropland through to the forest for areas underlain by dolomite. The contents of bulk SOC and its fractions were similar among the four vegetation types over limestone. The proportion of FLFC in bulk SOC was higher over dolomite than over limestone, but the case was inverse for the proportion of HFC, indicating SOC over limestone was more stable. However, the proportions of both FLFC and HFC were similar among the four vegetation types, implying that SOC stability was not changed by cropland conversion. Exchangeable calcium explained most of the variance of HFC content. Our study suggests that lithology not only affects SOC content and its stability, but modulates the dynamics of SOC fractions during post-agricultural succession.

Impacts of urban expansion on ecosystem services in the Beijing-Tianjin-Hebei urban agglomeration, China: A scenario analysis based on the Shared Socioeconomic Pathways

Understanding the impacts of urban expansion on ecosystem services (ESs) is important for sustainable development on regional and global scales. However, due to the uncertainty of future socioeconomic development and the complexity of urban expansion, assessing the impacts of future urban expansion on ESs remains challenging. In this study, we simulated the urban expansion in the Beijing-Tianjin-Hebei (BTH) urban agglomeration in China from 2013 to 2040, and assessed its potential impacts on ESs based on the Shared Socioeconomic Pathways (SSPs) and the Land Use Scenario Dynamics-urban (LUSD-urban) model. We found that urban land in the BTH urban agglomeration is expected to increase from 7605.25km2 in 2013 to 9401.75–11,936.00km2 in 2040. With continuing urban expansion, food production (FP), carbon storage (CS), water retention (WR), and air purification (AP) will decrease by 1.34–3.16%, 0.68–1.60%, 0.80–1.89%, and 0.37–0.87%, respectively. The conversion of cropland to urban land will be the main cause of ES losses. During 2013–2040, the losses of ESs caused by this conversion will account for 83.66–97.11% of the total losses in the whole region. Furthermore, the ES losses can cause considerable negative impacts on human well-being. The loss of FP will be equivalent to the food requirement of 3.68–8.61% of the total population in 2040, and the loss of CS will be 2.55–6.01% of the total standard coal consumption in 2013. To ensure sustainable development in the region, we suggest that effective policies and regulations should be implemented to protect cropland with high ES values from urban expansion.

Conversion of cropland to forage land and grassland increases soil labile carbon and enzyme activities in northeastern China

Abstract Soil labile carbon (C) and enzyme activities are valuable indicators of changes in soil quality and health. Understanding the changes in soil labile C and enzyme activities under different land uses is important to maintain soil quality and health and for sustainable land use. The primary objective of this study was to investigate the short-term influences of different land uses on SOC, soil labile C and enzyme activities in semiarid alkaline grassland of northeastern China. The experiment was organized as a block design with four replications of each land use treatment. Land use treatments were corn cropland (Corn), alfalfa forage land (Alfalfa), Lyemus chinensis grassland (AG), Lyemus chinensis grassland for mowing (AG + Mow) and restored grassland (RG), which were applied for five years. Total soil organic carbon (SOC), three labile C pools (oxidizable labile C; water-extractable organic C; microbial biomass C) and the activities of four soil enzymes (catalase; urease; alkaline phosphatase; invertase) were determined at the 0–20 cm depth in the five land use treatments. Results showed that soil labile C and enzyme activities were sensitive indicators of land use change. Conversion of cropland to forage land and grassland increased SOC (40.42%), soil labile C measures (25.50%) and enzyme activities (55.60%). However, the responses of different forms of soil labile C and enzyme activities to different land uses were not similar. Under Corn, AG + Mow, AG and RG land uses, the geometric means of labile C (27.01%, 10.95%, 17.52% and 5.11%, respectively) and enzyme activities (40.92%, 13.54%, 11.38% and 7.38%, respectively) were lower than those under Alfalfa, demonstrating that soil labile C and enzyme activities improved more under Alfalfa than under other land uses in northeastern China. Significant correlations were also obtained between SOC, soil labile C measures and enzyme activities. To conclude, soil labile C and enzyme activities can be expected to gradually increase with the conversion of cropland to grasslands and forage land, and planting to alfalfa offers a profitable and sustainable solution to our requirement for pairing forage production with rapid restoration of soil quality in the areas in which soils are not suitable for growing crops in the Songnen Grassland.

Effects of vegetation restoration on soil organic carbon in China: A meta-analysis

Vegetation restoration has been proposed as an effective method for increasing both plant biomass and soil carbon (C) stocks. In this study, 204 publications (733 observations) were analyzed, focusing on the effects of vegetation restoration on soil organic carbon (SOC) in China. The results showed that SOC was increased by 45.33%, 24.43%, 30.29% and 27.98% at soil depths of 0–20 cm, 20–40 cm, 40–60 cm and > 60 cm after vegetation restoration, respectively. Restoration from both cropland and non-cropland increased the SOC content. The conversion of non-cropland was more efficient in SOC accumulation than the conversion of cropland did, especially in > 40 cm layers. In addition, the conversion to planted forest led to greater SOC accumulation than that to other land use did. Conversion period and initial SOC content extended more influence on soil C accumulation as the main factors after vegetation restoration than temperature and precipitation did. The SOC content significantly increased with restoration period after long-term vegetation restoration (> 40 yr), indicating a large potential for further accumulation of carbon in the soil, which could mitigate climate change in the near future.

Land use change implications for large-scale cultivation of algae feedstocks in the United States Gulf Coast

Algae is considered a promising future feedstock for biofuels. Although several studies have been conducted to assess the environmental impact of algae-based fuels, land use change is one area that is commonly overlooked in previous life cycle assessment studies. However, land use change can impact the life cycle greenhouse gas (GHG) emissions of algal biofuels when large tracts of land are converted to algal raceway cultivation systems. This study assesses the impacts of land use change through a variety of means. The Intergovernmental Panel on Climate Change (IPCC) Tier 1 methodology was utilized to assess potential emissions resulting from the conversion of potential algae facility sites in the U.S. Gulf Coast, consisting of grassland, cropland, and forestland in several management conditions. These emission values over a 20-year time horizon were combined with guidance on promising sites for algae raceway development to provide an estimate of industry-wide GHG emissions impacts due to direct land use change (LUC). Direct LUC impacts appear to be important, with average GHG emissions of between 4 and 8 g CO2eq/MJ for grassland and cropland conversion, which is roughly 6.3% and 12.5% of the total GHG emission over the entire algae renewable diesel life cycle without considering the LUC. Emissions due to direct LUC could be even larger if previously forested lands are cleared, averaging 24.7 g CO2eq/MJ across a range of potential algae sites. This article details the methods, assumptions and initial LCA results for these land use change scenarios when considering the algae biofuels life cycle. Results from this LCA can help decision-makers recognize the importance of facility siting in overall environmental performance, and select locations of algae cultivation facilities to minimize direct LUC emissions.

Land management strategies for improving water quality in biomass production under changing climate

The Corn Belt states are the largest corn-production areas in the United States because of their fertile land and ideal climate. This attribute is particularly important as the region also plays a key role in the production of bioenergy feedstock. This study focuses on potential change in streamflow, sediment, nitrogen, and phosphorus due to climate change and land management practices in the South Fork Iowa River (SFIR) watershed, Iowa. The watershed is covered primarily with annual crops (corn and soybeans). With cropland conversion to switchgrass, stover harvest, and implementation of best management practices (BMPs) (such as establishing riparian buffers and applying cover crops), significant reductions in nutrients were observed in the SFIR watershed under historical climate and future climate scenarios. Under a historical climate scenario, suspended sediment (SS), total nitrogen (N), and phosphorus (P) at the outlet point of the SFIR watershed could decrease by up to 56.7%, 32.0%, and 16.5%, respectively, compared with current land use when a portion of the cropland is converted to switchgrass and a cover crop is in place. Climate change could cause increases of 9.7% in SS, 4.1% in N, and 7.2% in P compared to current land use. Under future climate scenarios, nutrients including SS, N, and P were reduced through land management and practices and BMPs by up to 54.0% (SS), 30.4% (N), and 7.1% (P). Water footprint analysis further revealed changes in green water that are highly dependent on land management scenarios. The study highlights the versatile approaches in landscape management that are available to address climate change adaptation and acknowledged the complex nature of different perspectives in water sustainability. Further study involving implementing landscape design and management by using long-term monitoring data from field to watershed is necessary to verify the findings and move toward watershed-specific regional programs for climate adaptation.

Impacts of vegetation restoration strategies on soil organic carbon and nitrogen dynamics in a karst area, southwest China

Soil carbon (C) sequestration through cropland conversion has been regarded as a major strategy to absorb atmospheric CO2 and thus mitigate global warming, but much uncertainty still exists in terms of restoration strategies. In this study, soil C and nitrogen (N) were measured 13 years after cropland conversion in a karst area, southwest China. Four restoration strategies were included, i.e., (i) restoration with an economic tree species Toona sinensis (TS), (ii) restoration with Guimu-1 hybrid elephant grass (GG), (iii) restoration with a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG), and iv) spontaneous regeneration (SR). Cropland under maize-soybean rotation (CR) was used as reference and the space-for-time substitution approach was adopted to evaluate soil C and N stock (0–15cm) change. Soil C stocks in TS and SR were elevated by 40% and 50%, respectively, relative to CR, while those in GG and ZG were not significantly changed. Soil N stocks in GG were not significantly changed, but stocks in TS, ZG and SR were enhanced by 130%, 81% and 117%, respectively, relative to CR. Significant correlation was found between soil C and N stock changes with rate of relative N stock change greater than that of C stock change. Similar to soil N stock, nitrate concentration, net nitrification rate and δ15N values in TS, ZG and SR were greater than those in GG or CR. Stepwise multiple linear regression indicated that exchangeable calcium was the soil variable mainly responsible for the dynamics of both soil C and N. Our results indicate that plantation with economic tree species and spontaneous regeneration are likely equally effective in soil C sequestration.

Accumulation of soil organic carbon after cropland conversion to short‐rotation willow and poplar

AbstractThe demand for bioenergy has increased the interest in short‐rotation woody crops (SRWCs) in temperate zones. With increased litter input and ceased annual soil cultivation, SRWC plantations may become soil carbon sinks for climate change mitigation. A chronosequence of 26 paired plots was used to study the potential for increasing soil organic carbon (SOC) under SRWC willow and poplar after conversion from cropland (CR) on well‐drained soils. We estimated SOC stocks in SRWC stands and adjacent CR and related the difference to time since conversion, energy crop species, SOC stock of the adjacent CR (proxy for initial SOC of SRWC) and the fine soil percentage (<63 μm) (FS). Soil cores to 40 cm depth were sampled and separated by layers of fixed depths (0–5, 5–10, 10–15, 15–25 and 25–40 cm). Additionally, soils were sampled from soil pits by genetic horizons to 100 cm depth. Comparisons of SOC stocks by equivalent soil masses showed that mean SOC stocks in SRWC were 1.7 times higher than those of CR in the top 5 cm of the soil (P < 0.001). The differences between SRWC and CR remained significant for the plough layer (0–25 cm) by a factor of 1.2 (P = 0.003), while no changes were detectable for the 0–40 cm (P = 0.32), or for the entire 0–100 cm soil layer (P = 0.29). The SOC stock ratio, that is the ratio of SOC stock in SRWC relative to CR, did not change significantly with time since conversion, although there was a tendency to an increase over time for the top 40 cm (P = 0.09). The SOC stock ratio was negatively correlated to SOC in CR and FS percentage, but there was no significant difference between willow and poplar at any depth. Our results suggest that SOC stocks in the plough layer increase after conversion to SRWC.

Forest carbon emissions from cropland expansion in the Brazilian Cerrado biome

Land use, land use change, and forestry accounted for two-thirds of Brazil’s greenhouse gas emissions profile in 2005. Amazon deforestation has declined by more than 80% over the past decade, yet Brazil’s forests extend beyond the Amazon biome. Rapid expansion of cropland in the neighboring Cerrado biome has the potential to undermine climate mitigation efforts if emissions from dry forest and woodland conversion negate some of the benefits of avoided Amazon deforestation. Here, we used satellite data on cropland expansion, forest cover, and vegetation carbon stocks to estimate annual gross forest carbon emissions from cropland expansion in the Cerrado biome. Nearly half of the Cerrado met Brazil’s definition of forest cover in 2000 (≥0.5 ha with ≥10% canopy cover). In areas of established crop production, conversion of both forest and non-forest Cerrado formations for cropland declined during 2003–2013. However, forest carbon emissions from cropland expansion increased over the past decade in Matopiba, a new frontier of agricultural production that includes portions of Maranhão, Tocantins, Piauí, and Bahia states. Gross carbon emissions from cropland expansion in the Cerrado averaged 16.28 Tg C yr−1 between 2003 and 2013, with forest-to-cropland conversion accounting for 29% of emissions. The fraction of forest carbon emissions from Matopiba was much higher; between 2010–2013, large-scale cropland conversion in Matopiba contributed 45% of total Cerrado forest carbon emissions. Carbon emissions from Cerrado-to-cropland transitions offset 5%–7% of the avoided emissions from reduced Amazon deforestation rates during 2011–2013. Comprehensive national estimates of forest carbon fluxes, including all biomes, are critical to detect cross-biome leakage within countries and achieve climate mitigation targets to reduce emissions from land use, land use change, and forestry.

Soil moisture decline due to afforestation across the Loess Plateau, China

The Loess Plateau of China is a region with one of the most severe cases of soil erosion in the world. Since the 1950s, there has been afforestation measure to control soil erosion and improve ecosystem services on the plateau. However, the introduction of exotic tree species (e.g., R. pseudoacacia, P. tabulaeformis and C. korshinskii) and high-density planting has had a negative effect on soil moisture content (SMC) in the region. Any decrease in SMC could worsen soil water shortage in both the top and deep soil layers, further endangering the sustainability of the fragile ecosystem. This study analyzed the variations in SMC following the conversion of croplands into forests in the Loess Plateau. SMC data within the 5-m soil profile were collected at 50 sites in the plateau region via field survey, long-term in-situ observations and documented literature. The study showed that for the 50 sites, the depth-averaged SMC was much lower under forest than under cropland. Based on in-situ measurements of SMC in agricultural plots and C. korshinskii plots in 2004–2014, SMC in the 0–4m soil profile in both plots declined significantly (p<0.01) during the growing season. The rate of decline in SMC in various soil layers under C. korshinskii plots (−0.008 to −0.016cm3cm−3yr−1) was much higher than those under agricultural plots (−0.004 to −0.005 cm3cm−3yr−1). This suggested that planting C. korshinskii intensified soil moisture decline in China’s Loess Plateau. In the first 20–25yr of growth, the depth-averaged SMC gradually decreased with stand age in R. pseudoacacia plantation, but SMC somehow recovered with increasing tree age over the 25-year period. Irrespectively, artificial forests consumed more deep soil moisture than cultivated crops in the study area, inducing soil desiccation and dry soil layer formation. Thus future afforestation should consider those species that use less water and require less thinning for sustainable soil conservation without compromising future water resources demands in the Loess Plateau.

晋西黄土区退耕还林22年后林地土壤物理性质的变化

晋西黄土区退耕还林22年后林地土壤物理性质的变化