Long-term tillage effects on soil organic carbon and dissolved organic carbon in a purple paddy soil of Southwest China

The impact of conservation tillage practices on soil carbon has been of great interest in recent years. Conservation tillage might have the potential to enhance soil carbon accumulation and alter the depth distribution of soil carbon compared to conventional tillage based systems. Changes in the soil organic carbon (SOC) as influenced by tillage, are more noticeable under long-term rather than short-term tillage practices. The objective of this study was to determine the impacts of long-term tillage on SOC and dissolved organic carbon (DOC) status after 19 years of four tillage treatments in a Hydragric Anthrosol. In this experiment four tillage systems included conventional tillage with rotation of rice and winter fallow system (CTF), conventional tillage with rotation of rice and rape system (CTR), no-till and ridge culture with rotation of rice and rape system (NT) and tillage and ridge culture with rotation of rice and rape system (TR). Soils were sampled in the spring of 2009 and sectioned into 0–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm depth, respectively.Tillage effect on SOC was observed, and SOC concentrations were much larger under NT than the other three tillage methods in all soil depths from 0 to 60 cm. The mean SOC concentration at 0–60 cm soil depth followed the sequence: NT (22.74 g kg^?1) > CTF (14.57 g kg^?1) > TR (13.10 g kg^?1) > CTR (11.92 g kg^?1). SOC concentrations under NT were significantly higher than TR and CTR (P < 0.01), and higher than CTF treatment (P < 0.05). The SOC storage was calculated on equivalent soil mass basis. Results showed that the highest SOC storage at 0–60 cm depth presented in NT, which was 158.52 Mg C ha^?1, followed by CTF (106.74 Mg C ha^?1), TR (93.11 Mg C ha^?1) and CTR (88.60 Mg C ha^?1). Compared with conventional tillage (CTF), the total SOC storage in NT increased by 48.51%, but decreased by 16.99% and 12.77% under CTR and TR treatments, respectively. The effect of tillage on DOC was significant at 0–10 cm soil layer, and DOC concentration was much higher under CTF than the other three treatments (P < 0.01). Throughout 0–60 cm soil depth, DOC concentrations were 32.92, 32.63, 26.79 and 22.10 mg kg^?1 under NT, CTF, CTR and TR, and the differences among the four treatments were not significant (P > 0.05). In conclusion, NT increased SOC concentration and storage compared to conventional tillage operation but not for DOC.     

Nitrogen and harvest effects on soil properties under rainfed switchgrass and no‐till corn over 9 years: implications for soil quality

Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long‐term effects of harvest timing and residue removal remain relatively unknown. Compared to no‐tilled corn (NT‐C, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB‐C), and soil aggregation] due to its perennial nature and deep‐rooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9 years for a rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha^?1) and harvest management on switchgrass (harvested in August and postfrost) and NT‐C (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB‐C, bulk density (BD), pH, P and K in the top 0–30 cm. Both NT‐C and switchgrass increased SMB‐C, SOC content, and aggregate stability over the 9 years, reflecting improvement from previous conventional management. However, the soils under switchgrass had double the percent aggregate stability, 1.3 times more microbial biomass, and a 5–8% decrease in bulk density in the 0–5 and 5–10 cm depths compared to NT‐C. After 9 years, cumulative decrease in available P was significantly greater beneath NT‐C (?24.0 kg P ha^?1) compared to switchgrass (?5.4 kg P ha^?1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (ΔSQI) in all depths. NT‐C without residue removal did not affect ΔSQI, but 50% residue removal decreased ΔSQI (0–30 cm) due to reduced aggregate stability and SMB‐C. Even with best‐management practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long‐term N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.     

Comparison of Polygonum aviculare L. seedling survival under different tillage systems in Mediterranean dryland agroecosystems

Weed community shifts in agroecosystems are influenced by multiple factors. Among them, tillage and crop rotation are very important. Polygonum aviculare survival at early plant stages and biomass and density at harvest time were compared under three tillage systems (conventional, CT; minimum, MT; and no tillage, NT). Field studies were conducted from 1997–1998 to 2000–2001, during a crop rotation (pea–wheat–wheat–barley). The layout of the tillage systems was not randomized, which led to confusion between the tillage effect and the site effect, although all three tillage systems were implemented as of 1993–1994 and the same agricultural practices had been employed in the entire field between 1981–1982 and 1992–1993. Seedling mortality was analysed in two monitored cohorts (2000 and 2001) using a generalized linear model of binomial probability distribution with a complementary log–log link function. Analyses of variance considering tillage system (or site) and block were performed on: (1) aboveground biomass at the harvest time of the four crops; (2) density and mean plant biomass at the end of the first two crops; (3) seedling density registered twice during the 1998–1999 campaign. The expected changes to estimated mortality showed that seedlings under NT had greater probabilities of failure than those growing under the other two tillage systems. These differences were found considering the tillage system apart from the crop and the accumulated precipitation effects, which also strongly affects seedling survival. Density, 1999–2000 biomass and 2000–2001 biomass were different under different tillage systems (P < 0.05), the lowest density occurring under NT and the lowest values of biomass under MT. Mean seedling densities were similar between CT and MT, but both were higher than densities under NT.     

Effects of field experimental warming on wheat root distribution under conventional tillage and no‐tillage systems

Despite the obvious importance of roots to agro‐ecosystem functioning, few studies have attempted to examine the effects of warming on root biomass and distribution, especially under different tillage systems. In this study, we performed a field warming experiment using infrared heaters on winter wheat, in long‐term conventional tillage and no‐tillage plots, to determine the responses of root biomass and distribution to warming. Soil monoliths were collected from three soil depths (0–10, 10–20, and 20–30 cm). Results showed that root biomass was noticeably increased under both till and no‐till tillage systems (12.1% and 12.9% in 2011, and 9.9% and 14.5% in 2013, in the two tillage systems, respectively) in the 0–30 cm depth, associated with a similar increase in shoot biomass. However, warming‐induced root biomass increases occurred in the deeper soil layers (i.e., 10–20 and 20–30 cm) in till, while the increase in no‐till was focused in the surface layer (0–10 cm). Differences in the warming‐induced increases in root biomass between till and no‐till were positively correlated with the differences in soil total nitrogen (R² = .863, p < .001) and soil bulk density (R² = .853, p < .001). Knowledge of the distribution of wheat root in response to warming should help manage nutrient application and cycling of soil C‐N pools under anticipated climate change conditions.     

Plastic pollution in croplands threatens long‐term food security

Plastic pollution is a global concern given its prevalence in aquatic and terrestrial ecosystems. Studies have been conducted on the distribution and impact of plastic pollution in marine ecosystems, but little is known on terrestrial ecosystems. Plastic mulch has been widely used to increase crop yields worldwide, yet the impact of plastic residues in cropland soils to soil health and crop production in the long term remained unclear. In this paper, using a global meta‐analysis, we found that the use of plastic mulch can indeed increase crop yields on average by 25%–42% in the immediate season due to the increase of soil temperature (+8%) and moisture (+17%). However, the unabated accumulation of film residues in the field negatively impacts its physicochemical properties linked to healthy soil and threatens food production in the long term. It has multiple negative impacts on plant growth including crop yield (at the mean rate of ?3% for every additional 100 kg/ha of film residue), plant height (?2%) and root weight (?5%), and soil properties including soil water evaporation capacity (?2%), soil water infiltration rate (?8%), soil organic matter (?0.8%) and soil available phosphorus (?5%) based on meta‐regression. Using a nationwide field survey of China, the largest user of plastic mulch worldwide, we found that plastic residue accumulation in cropland soils has reached 550,800 tonnes, with an estimated 6%–10% reduction in cotton yield in some polluted sites based on current level of plastic residue content. Immediate actions should be taken to ensure the recovery of plastic film mulch and limit further increase in film residue loading to maintain the sustainability of these croplands.     

Conservation agriculture, increased organic carbon in the top-soil macro-aggregates and reduced soil CO2 emissions

Background and aims

Conservation agriculture, the combination of minimal soil movement (zero or reduced tillage), crop residue retention and crop rotation, might have the potential to increase soil organic C content and reduce emissions of CO₂.

Methods

Three management factors were analyzed: (1) tillage (zero tillage (ZT) or conventional tillage (CT)), (2) crop rotation (wheat monoculture (W), maize monoculture (M) and maize-wheat rotation (R)), and (3) residue management (with (+r), or without (?r) crop residues). Samples were taken from the 0–5 and 5–10?cm soil layers and separated in micro-aggregates (< 0.25?mm), small macro-aggregates (0.25 to 1?mm) and large macro-aggregates (1 to 8?mm). The carbon content of each aggregate fraction was determined.

Results

Zero tillage combined with crop rotation and crop residues retention resulted in a higher proportion of macro-aggregates. In the 0–5?cm layer, plots with a crop rotation and monoculture of maize and wheat in ZT+r had the greatest proportion of large stable macro-aggregates (40%) and highest mean weighted diameter (MWD) (1.7?mm). The plots with CT had the largest proportion of micro-aggregates (27%). In the 5–10?cm layer, plots with residue retention in both CT and ZT (maize 1?mm and wheat 1.5?mm) or with monoculture of wheat in plots under ZT without residues (1.4?mm) had the greatest MWD. The 0–10?cm soil layer had a greater proportion of small macroaggregates compared to large macro-aggregates and micro-aggregates. In the 0–10?cm layer of soil with residues retention and maize or wheat, the greatest C content was found in the small and large macro-aggregates. The small macro-aggregates contributed most C to the organic C of the sample. For soil cultivated with maize, the CT treatments had significantly higher CO₂ emissions than the ZT treatments. For soil cultivated with wheat, CTR-r had significantly higher CO₂ emissions than all other treatments.

Conclusion

Reduction in soil disturbance combined with residue retention increased the C retained in the small and large macro-aggregates of the top soil due to greater aggregate stability and reduced the emissions of CO₂ compared with conventional tillage without residues retention and maize monoculture (a cultivation system normally used in the central highlands of Mexico).     

不同耕作措施对冬小麦-夏玉米复种连作系统土壤有机碳和水分利用效率的影响

在连续8年田间定位试验的基础上,分析了关中平原冬小麦 夏玉米复种连作系统2008—2009年连续两个生长季期间不同耕作措施(结合秸秆还田和不还田)对土壤有机碳和水分利用率的影响.结果表明: 相对于传统耕作,保护性耕作有利于土壤有机碳、水分利用效率和作物产量的提高,其中在“深松+秸秆还田”耕作模式下的增幅最高,土壤有机碳含量在0～30 cm土层增幅达到19.5%,水分利用效率和作物产量提高了16.9%和20.5%,而免耕模式则有效提高了0～10 cm土层有机碳含量.在该地区土壤和气候条件下,深松结合秸秆粉碎还田是最理想的耕作模式,最有利于土壤有机碳累积,并提高水分利用效率和作物产量.     

Tillage effects on carbon footprint and ecosystem services of climate regulation in a winter wheat–summer maize cropping system of the North China Plain

Inappropriate farm practices can increase greenhouse gases (GHGs) emissions and reduce soil organic carbon (SOC) sequestration, thereby increasing carbon footprints (CFs), jeopardizing ecosystem services, and affecting climate change. Therefore, the objectives of this study were to assess the effects of different tillage systems on CFs, GHGs emissions, and ecosystem service (ES) values of climate regulation and to identify climate-resilient tillage practices for a winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) cropping system in the North China Plain (NCP). The experiment was established in 2008 involving no-till with residue retention (NT), rotary tillage with residue incorporation (RT), sub-soiling with residue incorporation (ST), and plow tillage with residue incorporation (PT). The results showed that GHGs emissions from agricultural inputs were 6432.3–6527.3 kg CO₂-eq ha⁻¹ yr⁻¹ during the entire growing season, respectively. The GHGs emission from chemical fertilizers and irrigation accounted for >80% of that from agricultural inputs during the entire growing season. The GHGs emission from agricultural inputs were >2.3 times larger in winter wheat than that in the summer maize season. The CFs at yield-scale during the entire growing season were 0.431, 0.425, 0.427, and 0.427 without and 0.286, 0.364, 0.360, and 0.334 kg CO₂-eq kg⁻¹ yr⁻¹ with SOC sequestration under NT, RT, ST, and PT, respectively. Regardless of SOC sequestration, the CFs of winter wheat was larger than that of summer maize. Agricultural inputs and SOC change contributed mainly to the component of CFs of winter wheat and summer maize. The ES value of climate regulation under NT was ¥159.2, 515.6, and 478.1 ha⁻¹ yr⁻¹ higher than that under RT, ST, and PT during the entire growing season. Therefore, NT could be a preferred “Climate-resilient” technology for lowering CFs and enhancing ecosystem services of climate regulation for the winter wheat–summer maize system in the NCP.     

Acidification of Soil in a Dry Land Winter Wheat-sorghum/corn-fallow Rotation in the Semiarid U.S. Great Plains

Soil pH is decreasing in many soils in the semiarid Great Plains of the United States under dry land no-till (NT) cropping systems. This study was conducted to determine the rate of acidification and the causes of the acidification of a soil cropped to a winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]/corn (Zea mays L.)-fallow rotation (W-S/C-F) under NT. The study was conducted from 1989 to 2003 on soil with a long-term history of either continuous NT management [NT(LT)] (1962–2003) or conventional tillage (CT) (1962–1988) then converted to NT [NT(C)] (1989–2003). Nitrogen was applied as ammonium nitrate (AN) at a rate of 23 kg N ha⁻¹ in 1989 and as urea ammonium nitrate (UAN) at an average annual rate of 50 kg N ha⁻¹ from 1990 to 2003 for both NT treatments. Soil samples were collected at depth increments of 0–5, 5–10, 10–15, and 15–30 cm in the spring of 1989 and 2003. Acidification rates for the NT(LT) and NT(C) treatments were 1.13 and 1.48 kmol H⁺ ha⁻¹ yr⁻¹ in the 0–30 cm depth, respectively. The amount of CaCO₃ needed to neutralize the acidification is 57 and 74 kg ha⁻¹ yr⁻¹ for the NT(LT) and NT(C) treatments, respectively. A proton budget estimated by the Helyar and Porter [1989, Soil Acidity and Plant Growth, Academic Press] method indicated that NO₃⁻ leaching from the 30 cm depth was a primary cause of long-term acidification in this soil. Nitrate leaching accounted for 59 and 66% of the H⁺ from the acid causing factors for NT(LT) and NT(C) treatments, respectively. The addition of crop residues to the soil neutralized 62 and 47% of the acidity produced from the leaching of NO₃⁻, and 37 and 31% of the acid resulting from NO₃⁻ leaching and the other acid-causing constituents for the NT(LT) and NT(C) treatments, respectively. These results document that surface soils in dry land W-S/C-F rotations under NT are acidifying under current management practices. Improved management to increase nitrogen uptake efficiency from applied fertilizer would help reduce the rate of acidification. The addition of lime materials to prevent negative impacts on grain yields may be necessary in the future under current management practices. A contribution of the university of Nebraska Agricultural Research Division, Lincoln, NE 68583. Journal series No. 15120     

Dryland residue and soil organic matter as influenced by tillage, crop rotation, and cultural practice

Novel management practices are needed to increase dryland soil organic matter and crop yields that have been declining due to long-term conventional tillage with spring wheat (Triticum aestivum L.)-fallow system in the northern Great Plains, USA. The effects of tillage, crop rotation, and cultural practice were evaluated on dryland crop biomass (stems + leaves) yield, surface residue, and soil organic C (SOC) and total N (STN) at the 0?C20?cm depth in a Williams loam (fine-loamy, mixed, superactive, frigid, Typic Argiustolls) from 2004 to 2007 in eastern Montana, USA. Treatments were two tillage practices [no-tillage (NT) and conventional tillage (CT)], four crop rotations [continuous spring wheat (CW), spring wheat-pea (Pisum sativum L.) (W-P), spring wheat-barley (Hordeum vulgaris L.) hay-pea (W-B-P), and spring wheat-barley hay-corn (Zea mays L.)-pea (W-B-C-P)], and two cultural practices [regular (conventional seed rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height) and ecological (variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height)]. Crop biomass and N content were 4 to 44% greater in W-B-C-P than in CW in 2004 and 2005 and greater in ecological than in regular cultural practice in CT. Soil surface residue amount and C and N contents were greater in NT than in CT, greater in CW, W-P, and W-B-C-P than in W-B-P, and greater in 2006 and 2007 than in 2004 and 2005. The SOC and STN concentrations at 0?C5?cm were 4 to 6% greater in CW than in W-P or W-B-P in NT and CT from 2005 and 2007. In 2007, SOC content at 10?C20?cm was greater in W-P and W-B-P than in W-B-C-P in CT but STN was greater in W-B-P and W-B-C-P than in CW in NT. From 2004 to 2007, SOC and STN concentrations varied at 0?C5?cm but increased at 5?C20?cm. Diversified crop rotation and delayed planting with higher seed rates and banded N fertilization increased the amount of crop biomass returned to the soil and surface residue C and N. Although no-tillage increased surface residue C and N, continuous nonlegume cropping increased soil C and N levels at the surface layer compared with other crop rotations. Continued return of crop residue from 2004 to 2007 may increase soil C and N levels but long-term studies are needed to better evaluate the effect of management practices on soil C and N levels under dryland cropping systems in the northern Great Plains.     

保护性耕作对陇中旱作农田水分特征的影响

陇中旱农区生产力水平低而不稳,而保护性耕作措施是农业可持续发展的重要途径.本研究依托2001年建立在陇中旱农区的长期不同耕作措施的定位试验,研究了不同耕作措施对土壤水分入渗、蒸发、作物产量和水分利用效率的影响.该试验共设6个处理,分别为传统耕作(T)、免耕秸秆覆盖(NTS)、免耕(NT)、传统耕作+秸秆翻入(TS)、传统耕作+覆膜(TP)、免耕覆膜(NTP),春小麦和豌豆年间轮作.结果表明:与T处理相比, NTS处理的小麦地和豌豆地的土壤容重显著降低,总孔隙度显著增加.保护性耕作措施降低了豌豆地0～5 cm土壤渗吸率,NTS处理渗吸率比T处理降低56.2%.保护性耕作提高了土壤饱和导水率,无论小麦地和豌豆地,NTS均比T处理显著提高了饱和导水率,增幅为52.8%～107.1%.保护性耕作显著降低了作物生育期棵间蒸发量,NTP、TP、NTS比T处理降低了14.4%～50.8%,并减弱了雨后土壤蒸发.保护性耕作提高了作物产量和水分利用效率,NTS、TP、NTP的产量比传统耕作提高了9.5%～62.8%,水分利用效率比传统耕作提高了0.4%～50.9%.因此,在陇中旱农区,保护性耕作措施可以提高水分利用效率,增加作物产量.     

Conventional tillage decreases the abundance and biomass of earthworms and alters their community structure in a global meta‐analysis

The adoption of less intensive soil cultivation practices is expected to increase earthworm populations and their contributions to ecosystem functioning. However, conflicting results have been reported on the effects of tillage intensity on earthworm populations, attributed in narrative reviews to site‐dependent differences in soil properties, climatic conditions and agronomic operations (e.g. fertilization, residue management and chemical crop protection). We present a quantitative review based on a global meta‐analysis, using paired observations from 165 publications performed over 65 years (1950–2016) across 40 countries on five continents, to elucidate this long‐standing unresolved issue. Results showed that disturbing the soil less (e.g. no‐tillage and conservation agriculture [CA]) significantly increased earthworm abundance (mean increase of 137% and 127%, respectively) and biomass (196% and 101%, respectively) compared to when the soil is inverted by conventional ploughing. Earthworm population responses were more pronounced when the soil had been under reduced tillage (RT) for a long time (>10 years), in warm temperate zones with fine‐textured soils, and in soils with higher clay contents (>35%) and low pH (<5.5). Furthermore, retaining organic harvest residues amplified this positive response to RT, whereas the use of the herbicide glyphosate did not significantly affect earthworm population responses to RT. Additional meta‐analyses confirmed that epigeic and, more importantly, the bigger‐sized anecic earthworms were the most sensitive ecological groups to conventional tillage. In particular, the deep burrower Lumbricus terrestris exhibited the strongest positive response to RT, increasing in abundance by 124% more than the overall mean of all 13 species analysed individually. The restoration of these two important ecological groups of earthworms and their burrowing, feeding and casting activities under various forms of RT will ensure the provision of ecosystem functions such as soil structure maintenance and nutrient cycling by “nature's plough.”     

不同耕作方式下长期秸秆还田对旱作春玉米田土壤碳、氮、水含量及产量的影响

为了探索渭北旱塬春玉米田保护性轮耕模式的土壤培肥效果和增产增收效应,于2007—2014年在陕西合阳实施了秸秆覆盖或还田条件下免耕/深松(NT/ST)、深松/翻耕(ST/CT)、翻耕/免耕(CT/NT)、连续免耕(NT)、连续深松(ST)和连续翻耕(CT)等6种耕作处理田间定位试验,测定并分析了2010—2014年玉米收获期各耕作处理下0～60 cm土壤有机碳、氮储量,0～200 cm土层土壤含水量变化及春玉米产量差异.结果表明: 6种耕作处理中以NT/ST处理增加土壤有机碳和全氮储量最为明显.与2007年试验前相比,6种耕作处理均增加了0～60 cm土层土壤有机碳储量,5年平均值增幅为12.3%～28.3%,5种保护性耕作处理土壤有机碳储量5年平均值较CT对照处理显著增加7.1%～13.2%.NT/ST、ST/CT、CT/NT等3种轮耕处理和NT处理0～60 cm土层土壤氮储量5年平均值较试验前增加2.5%～7.3%.NT/ST、ST/CT、CT/NT、NT和ST处理土壤氮储量5年平均值比连续翻耕增加3.6%～11.1%.5种保护性耕作处理土壤含水量较CT处理依次增加5.7%、2.3%、2.0%、5.5%和4.4%,以NT/ST处理土壤含水量最高.6种耕作处理春玉米平均产量表现为NT/ST>ST/CT>ST >NT>CT/NT>CT,以NT/ST处理最高,分别较其他5种处理显著增产4.2%、13.0%、11.3%、4.7%和13.8%;经济效益平均表现为NT/ST>ST/CT>ST>NT>CT/NT>CT.在6种耕作处理中,免耕/深松轮耕处理在改善土壤环境质量、提升土壤肥力和增产增收方面都表现出优越性,为旱作春玉米田较适宜的土壤轮耕模式.     

Contribution of above‐ and belowground bioenergy crop residues to soil carbon

GHG mitigation by bioenergy crops depends on crop type, management practices, and the input of residue carbon (C) to the soil. Perennial grasses may increase soil C compared to annual crops because of more extensive root systems, but it is less clear how much soil C is derived from above‐ vs. belowground inputs. The objective of this study was to synthesize the existing knowledge regarding soil C inputs from above‐ and belowground crop residues in regions cultivated with sugarcane, corn, and miscanthus, and to predict the impact of residue removal and tillage on soil C stocks. The literature review showed that aboveground inputs to soil C (to 1‐m depth) ranged from 70% to 81% for sugarcane and corn vs. 40% for miscanthus. Modeled aboveground C inputs (to 30 cm depth) ranged from 54% to 82% for sugarcane, but were 67% for miscanthus. Because 50% of observed miscanthus belowground biomass is below 30 cm depth, it may be necessary to increase the depth of modeled soil C dynamics to reconcile modeled belowground C inputs with measured. Modeled removal of aboveground corn residue (25–100%) resulted in C stock reduction in areas of corn–corn–soybean rotation under conventional tillage, while no‐till management lessoned this impact. In sugarcane, soil C stocks were reduced when total aboveground residue was removed at one site, while partial removal of sugarcane residue did not reduce soil C stocks in either area. This study suggests that aboveground crop residues were the main C‐residue source to the soil in the current bioethanol sector (corn and sugarcane) and the indiscriminate removal of crop residues to produce cellulosic biofuels can reduce soil C stocks and reduce the environmental benefits of bioenergy. Moreover, a switch to feedstocks such as miscanthus with more allocation to belowground C could increase soil C stocks at a much faster rate.     

不同耕作方式下草甸栗钙土燕麦田土壤微生物特征

河北坝上地区高寒半干旱的独特生态环境,土地沙化风蚀严重,作物产量低,土壤微生物的活动尚未有深入研究,尤其是人为干扰下的土壤微生物在作物生长季的动态变化。为了明确耕作方式对草甸栗钙土土壤微生物性状的影响特征,依托定位12a的免耕、深松、常规耕作田间试验基础,通过辅助设置12a免耕、深松后的翻耕处理,监测了燕麦田土壤微生物量碳、活跃微生物量和土壤呼吸速率等性状。结果表明,土壤微生物量碳与活跃微生物量围绕燕麦抽穗期为"W型"时序变化,长期免耕与深松下呈现0—10 cm表层土壤富集微生物量碳的空间分布特征。免耕与深松有利于提高0—10 cm土层土壤微生物量,多年免耕和深松后翻耕能使土层间土壤活跃微生物量差异减小。燕麦田土壤呼吸速率呈现"U型"时序变化,免耕0—10 cm土层呼吸速率具有较其他耕作方式更高的趋势。在燕麦生育期内,土壤呼吸商一直处于较低而平稳的水平,燕麦收获后进入土壤微生物的高活性期;0—10 cm土层翻耕与多年免耕与深松后的翻耕处理土壤呼吸商有高于免耕处理的趋势。翻耕对于促进冷凉区土壤库存养分的活化与适时供应具有重要作用。     

Effects of Conservation Tillage on Topsoil Microbial Metabolic Characteristics and Organic Carbon within Aggregates under a Rice (Oryza sativa L.) –Wheat (Triticum aestivum L.) Cropping System in Central China

Investigating microbial metabolic characteristics and soil organic carbon (SOC) within aggregates and their relationships under conservation tillage may be useful in revealing the mechanism of SOC sequestration in conservation tillage systems. However, limited studies have been conducted to investigate the relationship between SOC and microbial metabolic characteristics within aggregate fractions under conservation tillage. We hypothesized that close relationships can exist between SOC and microbial metabolic characteristics within aggregates under conservation tillage. In this study, a field experiment was conducted from June 2011 to June 2013 following a split-plot design of a randomized complete block with tillage practices [conventional intensive tillage (CT) and no tillage (NT)] as main plots and straw returning methods [preceding crop residue returning (S, 2100−2500 kg C ha⁻¹) and removal (NS, 0 kg C ha^-1)] as subplots with three replications. The objective of this study was to reveal the effects of tillage practices and residue-returning methods on topsoil microbial metabolic characteristics and organic carbon (SOC) fractions within aggregates and their relationships under a rice–wheat cropping system in central China. Microbial metabolic characteristics investigated using the Biolog system was examined within two aggregate fractions (>0.25 and <0.25 mm). NT treatments significantly increased SOC concentration of bulk soil, >0.25 aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer by 5.8%, 6.8% and 7.9% relative to CT treatments, respectively. S treatments had higher SOC concentration of bulk soil (12.9%), >0.25 mm aggregate (11.3%), and <0.25 mm aggregate (14.1%) than NS treatments. Compared with CT treatments, NT treatments increased MBC by 11.2%, 11.5%, and 20%, and dissolved organic carbon (DOC) concentration by 15.5%, 29.5%, and 14.1% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer, respectively. Compared with NS treatments, S treatments significantly increased MBC by 29.8%, 30.2%, and 24.1%, and DOC concentration by 23.2%, 25.0%, and 37.5% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer, respectively. Conservation tillage (NT and S) increased microbial metabolic activities and Shannon index in >0.25 and <0.25 mm aggregates in the 0−5 cm soil layer. Redundancy analysis showed that the SOC and its fractions (DOC and MBC) were closely correlated with microbial metabolic activities. Structural equation modelling showed that the increase in microbial metabolic activities directly improved SOC by promoting DOC in >0.25 mm aggregate in the upper (0−5 cm) soil layer under conservation tillage systems, as well as directly and indirectly by promoting DOC and MBC in <0.25 mm aggregate. Our results suggested that conservation tillage increased SOC in aggregates in the topsoil by improving microbial metabolic activities.     

Methane and nitrous oxide emissions under no‐till farming in China: a meta‐analysis

No‐till (NT) practices are among promising options toward adaptation and mitigation of climate change. However, the mitigation effectiveness of NT depends not only on its carbon sequestration potential but also on soil‐derived CH₄ and N₂O emissions. A meta‐analysis was conducted, using a dataset involving 136 comparisons from 39 studies in China, to identify site‐specific factors which influence CH₄ emission, CH₄ uptake, and N₂O emission under NT. Comparative treatments involved NT without residue retention (NT0), NT with residue retention (NTR), compared to plow tillage (PT) with residue removed (PT0). Overall, NT0 significantly decreased CH₄ emission by ~30% (P < 0.05) compared to PT0 with an average emission 218.8 kg ha⁻¹ for rice paddies. However, the increase in N₂O emission could partly offset the benefits of the decrease in CH₄ emission under NT compared to PT0. NTR significantly enhanced N₂O emission by 82.1%, 25.5%, and 20.8% (P < 0.05) compared to PT0 for rice paddies, acid soils, and the first 5 years of the experiments, respectively. The results from categorical meta‐analysis indicated that the higher N₂O emission could be mitigated by adopting NT within alkaline soils, for long‐term duration, and with less N fertilization input when compared to PT0. In addition, the natural log (lnR) of response ratio of CH₄ and N₂O emissions under NT correlated positively (enhancing emission) with climate factors (temperature and precipitation) and negatively (reducing emission) with experimental duration, suggesting that avoiding excess soil wetness and using NT for a long term could enhance the benefits of NT. Therefore, a thorough understanding of the conditions favoring greenhouse gas(es) reductions is essential to achieving climate change mitigation and advancing food security in China.     

Long‐term no‐till and stover retention each decrease the global warming potential of irrigated continuous corn

Over the last 50 years, the most increase in cultivated land area globally has been due to a doubling of irrigated land. Long‐term agronomic management impacts on soil organic carbon (SOC) stocks, soil greenhouse gas (GHG) emissions, and global warming potential (GWP) in irrigated systems, however, remain relatively unknown. Here, residue and tillage management effects were quantified by measuring soil nitrous oxide (N₂O) and methane (CH₄) fluxes and SOC changes (ΔSOC) at a long‐term, irrigated continuous corn (Zea mays L.) system in eastern Nebraska, United States. Management treatments began in 2002, and measured treatments included no or high stover removal (0 or 6.8 Mg DM ha^?1 yr^?1, respectively) under no‐till (NT) or conventional disk tillage (CT) with full irrigation (n = 4). Soil N₂O and CH₄ fluxes were measured for five crop‐years (2011–2015), and ΔSOC was determined on an equivalent mass basis to ~30 cm soil depth. Both area‐ and yield‐scaled soil N₂O emissions were greater with stover retention compared to removal and for CT compared to NT, with no interaction between stover and tillage practices. Methane comprised <1% of total emissions, with NT being CH₄ neutral and CT a CH₄ source. Surface SOC decreased with stover removal and with CT after 14 years of management. When ΔSOC, soil GHG emissions, and agronomic energy usage were used to calculate system GWP, all management systems were net GHG sources. Conservation practices (NT, stover retention) each decreased system GWP compared to conventional practices (CT, stover removal), but pairing conservation practices conferred no additional mitigation benefit. Although cropping system, management equipment/timing/history, soil type, location, weather, and the depth to which ΔSOC is measured affect the GWP outcomes of irrigated systems at large, this long‐term irrigated study provides valuable empirical evidence of how management decisions can impact soil GHG emissions and surface SOC stocks.     

Differences in field‐scale N2O flux linked to crop residue removal under two tillage systems in cold climates

Residue removal for biofuel production may have unintended consequences for N₂O emissions from soils, and it is not clear how N₂O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field‐scale N₂O flux over 5 years (2005–2007, 2010–2011) from an annual crop rotation to evaluate how N₂O emissions are influenced by no‐till (NT) compared to conventional tillage (CV), and how crop residue removal (R?) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N₂O flux between tillage practices at the onset of the growing season, but CT had 1.4–6.3 times higher N₂O flux than NT overwinter. Nitrous oxide emissions were higher due to R? compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N₂O emissions. The potential for higher N₂O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event, dissolved organic C (DOC) was negatively correlated to peak N₂O flux (r = ?0.93). This suggests that lower N₂O emissions with R+ vs. R? may reflect more complete stepwise denitrification to N₂ during winter and possibly relate to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter.     

基于最小限制水分范围评价不同耕作方式对土壤有机碳的影响

以2009年吉林省德惠市中层黑土上进行了8a的田间定位试验小区土壤为研究对象,对免耕和秋翻两种耕作方式及玉米-大豆轮作和玉米连作两种种植方式下耕层有机碳进行分析,分别采用加权平均和分层两种方法计算最小限制水分范围(LLWR),用其评价不同耕作方式对土壤有机碳的影响.结果表明,免耕在玉米-大豆轮作和玉米连作下0-5 cm土壤有机碳含量分别比秋翻增加了15.2％和11.5％ (P＜0.05).采用加权平均法计算的LLWR值为0.148-0.166 cm3/cm3,不同耕作方式下玉米-大豆轮作的LLWR高于玉米连作且在两种种植方式下均表现出免耕小于秋翻的特点;利用分层法计算得到的LLWR值介于0.130-0.173 cm3/cm3之间,玉米-大豆轮作和玉米连作下免耕0-5 cm LLWR均显著小于秋翻,而5-30 cm LLWR数值免耕大于秋翻(P＞0.05);玉米-大豆轮作下0-30 cm各层LLWR均高于玉米连作.由于LLWR可以评价不同耕作方式对土壤有机碳的影响,因此采用加权平均法计算的LLWR可以客观的反映不同耕作处理尤其是种植方式对土壤有机碳的影响;而采用分层法计算的LLWR则更清晰的刻画了土壤表层与亚表层固碳能力的差异.