No abstract

By searching literature databases, we obtained more than 200 articles published since 1993 that related to the measurements of topsoil organic carbon (SOC) in different regions. To objectively evaluate the changes in the SOC over the last two decades, we selected 132 representative articles from these documented articles. More than sixty thousand soil samples and/or sampling sites were included in the selected articles. Results from analyzing these data sets indicated that the concentration of SOC increased in 53%-59%, decreased in 30%-31% and stabilized in 4%-6% of the national croplands, respectively. A further investigation showed that the total increment of SOC in Chinese croplands ranged from 311.3 Tg to 401.4 Tg. In terms of administrative region, significant increase occurred in eastern and northern China and decrease in northeastern China, respectively. When evaluated by soil great groups, the SOC increased considerably in paddy soils and fluvo-aquic soils and reduced conspicuously in black soils. The increase of SOC is attributed to the amendments of crop residues and organic manure, the augment of synthetic fertilizer application and the optimal combination of nutrients, and the development of no-tillage and reduced-tillage practice. Water loss and soil erosion and low input induced a great decrease of the SOC in black soils. In order to effectively enhance soil C sequestrations and to greatly control the SOC reduction in northeastern China, future efforts should be made in developing new techniques, training farmers and consummating the policy of governmental compensation, by which the application of crop straw, the improvement of fertilization, the practice of no-tillage and reduced-tillage, and the control of water loss and soil erosion could be further realized. To respond to the increasing pressure from the Kyoto Protocol thenceforward, four aspects were further addressed for future research needs, including the quantification of SOC storage in the Second State Soil Survey and at present, the understanding of control mechanisms in both anthropogenic and non-anthropogenic causes that determine SOC dynamics, the investigation of options that can effectively enhance SOC sequestration and/or reduce SOC loss, and the assessments of potentials and the likely SOC dynamics in the future on a national scale.

Conservation agriculture has been shown to have multiple benefits for soils, crop yield and the environment, and consequently, no-till, the central practice of conservation agriculture, has rapidly expanded. However, studies show that the potential for carbon (C) sequestration in no-till farming sometimes is not realized, let alone the ability to maintain or improve crop yield. Here we present a global analysis of no-till-induced changes of soil C and crop yield based on 260 and 1,970 paired studies; respectively. We show that, relative to local conventional tillage, arid regions can benefit the most from conservation agriculture by achieving a win-win outcome of enhanced C sequestration and increased crop yield. However, more humid regions are more likely to increase SOC only, while some colder regions have yield losses and soil C loss as likely as soil C gains. In addition to site-specific characteristics and management, a careful assessment of the regional climate is needed to determine the potential benefits of adopting conservation agriculture. K E Y W O R D S climate, conservation agriculture, crop yield, meta-analysis, soil organic carbon, win-win outcome See also the Commentary on this article by Hunt et al. 26, 3188-3189

No abstract

Considerable efforts have been made to assess the contribution of forest and grassland ecosystems to the global carbon budget, while less attention has been paid to agriculture. Net primary production (NPP) of Chinese croplands and driving factors are seldom taken into account in the regional carbon budget. We studied crop NPP by analyzing the documented crop yields from 1950 to 1999 on a provincial scale. Total NPP, including estimates of the aboveground and belowground components, was calculated from harvested yield data by (1) conversion from economic yield of the crop to aboveground mass using the ratio of aboveground residue production to the economic yield, (2) estimation of belowground mass as a function of aboveground mass, and (3) conversion from total dry mass to carbon mass. This approach was applied to 13 crops, representing 86.8% of the total harvested acreage of crops in China. Our results indicated that NPP in Chinese croplands increased markedly during this period. Averaging for each decade, the amount of NPP was 146 +/- 32, 159 +/- 34, 260 +/- 55, 394 +/- 85, and 513 +/- 111 Tg C/yr (mean +/- SD) in the 1950s, 1960s, 1970s, 1980s, and 1990s, respectively. This increase may be attributed to synthetic fertilizer application. A further investigation indicated that the climate parameters of temperature and precipitation determined the spatial variability in NPP. Spatiotemporal variability in NPP can be well described by the consumption of synthetic fertilizer and by climate parameters. In addition, the total amount of residue C and root C retained by the soils was estimated to be 618 Tg, with a range from 300 to 1040 Tg over the 50 years.

Cropland soil organic carbon (SOC) is undergoing substantial alterations due to both environmental and anthropogenic changes. Although numerous case studies have been conducted, there remains a lack of quantification of the consequences of such environmental and anthropogenic changes on the SOC sequestration across global agricultural systems. Here, we conducted a global meta-analysis of SOC changes under different fertilizer managements, namely unbalanced application of chemical fertilizers (UCF), balanced application of chemical fertilizers (CF), chemical fertilizers with straw application (CFS), and chemical fertilizers with manure application (CFM). We show that topsoil organic carbon (C) increased by 0.9 (0.7–1.0, 95% confidence interval (CI)) g kg−1 (10.0%, relative change, hereafter the same), 1.7 (1.2–2.3) g kg−1 (15.4%), 2.0 (1.9–2.2) g kg−1 (19.5%) and 3.5 (3.2–3.8) g kg−1 (36.2%) under UCF, CF, CFS and CFM, respectively. The C sequestration durations were estimated as 28–73 years under CFS and 26–117 years under CFM but with high variability across climatic regions. At least 2.0 Mg ha−1 yr−1 C input is needed to maintain the SOC in ~85% cases. We highlight a great C sequestration potential of applying CF, and adopting CFS and CFM is highly important for either improving or maintaining current SOC stocks across all agro–ecosystems.

No abstract

It has been well recognized that converting wetlands to cropland results in loss of soil organic carbon (SOC), while less attention was paid to concomitant changes in methane (CH 4 ) and nitrous oxide (N 2 O) emissions. Using datasets from the literature and field measurements, we investigated loss of SOC and emissions of CH 4 and N 2 O due to marshland conversion in northeast China. Analysis of the documented crop cultivation area indicated that 2.91 Mha of marshland were converted to cropland over the period 1950-2000. Marshland conversion resulted in SOC loss of $ 240 Tg and introduced $1.4 Tg CH 4 and $ 138 Gg N 2 O emissions in the cropland, while CH 4 emissions reduced greatly in the marshland, cumulatively $28 Tg over the 50 years. Taking into account the loss of SOC and emissions of CH 4 and N 2 O, the global warming potential (GWP) at a 20-year time horizon was estimated to be $ 180 Tg CO 2 _eq. yr À1 in the 1950s and $ 120 Tg CO 2 _eq. yr À1 in the 1990s, with a $ 33% reduction. When calculated at 100-year time horizon, the GWP was $ 73 Tg CO 2 _eq. yr À1 in the 1950s and $ 58 Tg CO 2 _eq. yr À1 in the 1990s, with a $ 21% reduction. It was concluded that marshland conversion to cropland in northeast China reduced the greenhouse effect as far as GWP is concerned. This reduction was attributed to a substantial decrease in CH 4 emissions from the marshland. An extended inference is that the declining growth rate of atmospheric CH 4 since the 1980s might be related to global loss of wetlands, but this connection needs to be confirmed.