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Greenhouse gas (GHG) emissions can be partially mitigated with conservation agriculture. In this study, we assessed the effects of conventional tillage (CT) and no-tillage (NT), as well as five NT cover crop-based cropping systems, on yield-scaled GHG emissions in two long-term experiments (18 and 19 years) on a subtropical Paleudult. Air samples collected in static closed chambers were used to measure nitrous oxide (N2O) and methane (CH4) fluxes. The annual rate of change in soil organic C from the beginning of the experiments was used as proxy for net CO2 flux. Cumulative annual emissions of the three GHG and the CO2 costs of agricultural inputs and operations were taken in full account when estimating the global warming potential (GWP). Under legume cover crops, NT soil exhibited increased N2O emissions relative to CT soil (531 vs 217 kg CO2eq ha−1 yr−1); however, emissions of this gas from NT soil were fully offset by CO2 retention in soil organic matter (−2063 to −3940 kg CO2 ha−1 yr−1). Soil CH4 fluxes were very low with all management systems (−1.5 to 30.5 kg CO2eq ha−1 yr−1). NT soil under legume cover crops behaved as a net sink for GHG (GWP ranged from −971 to −2818 kg CO2eq ha−1 yr−1); by contrast, CT soil and NT soil with a low biomass input were net sources of GHG (GWP ranged from 857 to 2133 kg CO2eq ha−1 yr−1). The legume cover crops increased maize yield and further reduced yield-scaled GHG emissions. This result suggests that conservation management practices involving no-till in combination with legume cover crops provide an effective approach to sustainable low-C footprint food production in subtropical regions.

Soil nitrous oxide (N2O) emissions are affected by management practices, but little information is available on the interactive effects of tillage, cropping systems and N sources in tropical and subtropical soils. In an 18-yr old experiment located in a subtropical Acrisol of Southern Brazil we conducted a sequence of two trials. The 1-year trial (October 2003–2004) was set to evaluate the long-term effects of tillage [CT: conventional; and NT: no-tillage] and cropping systems [O/M: black oat (Avena strigosa Schreb.)/maize (Zea mays L.); and V/M: vetch (Vicia sativa L.)/maize] on soil N2O emissions, either in the post-management period (45 days after desiccation and knife-rolling of winter cover crops) or in the whole year. The second and short-term trial (October–November 2004) was carried out to compare the impact of N sources [urea (mineral) and legume-residue of vetch (biologically fixed), both at 180 kg N ha−1] on soil N2O emissions during 53 days after cover-crop management. Air sampling was carried out by static chambers and N2O analysis by gas chromatography. In the 45-day post-management period of the 1-year trial, soil N2O emissions were practically not affected by tillage systems, but increased 4 times due to vetch residues (average of 0.40 ± 0.08 kg N ha−1 in V/M versus 0.10 ± 0.05 kg N ha−1 in O/M) and related with soil contents of NO3−-N, NH4+-N, and dissolved organic C (DOC). Over the whole year, soil N2O emissions under CT were similar for grass- and legume-based cropping systems and averaged 0.43 ± 0.17 kg N ha−1, while NT exacerbated N2O emissions in the legume-based cropping system (0.80 ± 0.07 kg N ha−1 in V/M versus −0.07 ± 0.06 kg N ha−1 in O/M). Maize yield was not affected by tillage, but increased from 2.32 Mg ha−1 in O/M to 4.44 Mg ha−1 in V/M. Yield-scaled N2O emissions varied from −33 g N2O-N Mg−1 grain in NT O/M to 179 g N2O-N Mg−1 grain in NT V/M, and were intermediate in CT soil (106 and 156 g N2O-N Mg−1grain in V/M and O/M cropping systems, respectively). In the short-term trial, the N2O emitted in excess relative to the control treatment (O/M without N fertilizer) was at least 3 times greater with urea-N (0.44% of applied N) than with legume-residue-Nsource (0.13% of applied N). Yield-scaled N2O emission after vetch residues management (67 g N Mg−1 grain) was half of that after urea-N application (152 g N Mg−1 grain). Partially supplying the maize N requirements with winter legume cover-crops may be a feasible strategy to mitigate soil N2O emissions in the subtropical conservation agriculture.

Methane (CH 4 ) emission from agricultural soils increases dramatically as a result of deleterious effect of soil disturbance and nitrogen fertilization on methanotrophic organisms; however, few studies have attempted to evaluate the potential of long-term conservation management systems to mitigate CH 4 emissions in tropical and subtropical soils. This study aimed to evaluate the long-term effect (>19 years) of no-till grass- and legume-based cropping systems on annual soil CH 4 fluxes in a formerly degraded Acrisol in Southern Brazil. Air sampling was carried out using static chambers and CH 4 analysis by gas chromatography. Analysis of historical data set of the experiment evidenced a remarkable effect of high C- and N-input cropping systems on the improvement of biological, chemical, and physical characteristics of this no-tilled soil. Soil CH 4 fluxes, which represent a net balance between consumption (−) and production (+) of CH 4 in soil, varied from −40 ± 2 to +62 ± 78 μg C m −2 h −1 . Mean weighted contents of ammonium (NH 4 + –N) and dissolved organic carbon (DOC) in soil had a positive relationship with accumulated soil CH 4 fluxes in the post-management period ( r 2 = 0.95, p = 0.05), suggesting an additive effect of these nutrients in suppressing CH 4 oxidation and stimulating methanogenesis, respectively, in legume-based cropping systems with high biomass input. Annual CH 4 fluxes ranged from −50 ± 610 to +994 ± 105 g C ha −1 , which were inversely related to annual biomass-C input ( r 2 = 0.99, p = 0.003), with the exception of the cropping system containing pigeon pea, a summer legume that had the highest biologically fixed N input (>300 kg ha −1 yr −1 ). Our results evidenced a small effect of conservation management systems on decreasing CH 4 emissions from soil, despite their significant effect restoring soil quality. We hypothesized that soil CH 4 uptake strength has been off-set by an injurious effect of biologically fixed N in legume-based cropping systems on soil methanotrophic microbiota, and by the methanogenesis increase as a result of the O 2 depletion in niches of high biological activity in the surface layer of the no-tillage soil.

The carbon management index (CMI) is derived from the total soil organic C pool and C lability and is useful to evaluate the capacity of management systems to promote soil quality. However, the CMI has not been commonly used for this purpose, possible due to some limitations of the 333 mM KMnO4-chemical oxidation method conventionally employed to determine the labile C fraction. We hypothesized, however, that physical fractionation of organic matter is an alternative approach to determine the labile C. The objectives of this study were (i) to assess the physical fractionation with density (NaI 1.8 Mg m−3) and particle-size separation (53 μm mesh) as alternative methods to the KMnO4-chemical oxidation (60 and 333 mM) in determining the labile C and thus the CMI, and (ii) to evaluate the capacity of long-term (19 years) no-till cropping systems (oat/maize: O/M, oat + vetch/maize: O + V/M, oat + vetch/maize + cowpea: O + V/M + C, and pigeon pea + maize: P + M) and N fertilization (0 and 180 kg N ha−1) to promote the soil quality of a Southern Brazilian Acrisol, using the CMI as the main assessment parameter. Soil samples were collected from 0 to 12.5 cm layer, and the soil of an adjacent native grassland was taken as reference. The mean annual C input of the cropping systems varied from 3.4 to 6.0 Mg ha−1 and the highest amounts occurred in legume-based cropping systems and N fertilized treatments. The C pool index was positively related to the annual C input (r2 = 0.93, P

Conservation management systems can improve soil organic matter stocks and contribute to atmospheric C mitigation. This study was carried out in a 18-year long-term experiment conducted on a subtropical Acrisol in Southern Brazil to assess the potential of tillage systems [conventional tillage (CT) and no-till (NT)], cropping systems [oat/maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/MC)] and N fertilization [0 kg N ha−1 year−1 (0 N) and 180 kg N ha−1 year−1 (180 N)] for mitigating atmospheric C. For that, the soil organic carbon (SOC) accumulation and the C equivalent (CE) costs of the investigated management systems were taken into account in comparison to the CT O/M 0 N used as reference system. No-till is known to produce a less oxidative environment than CT and resulted in SOC accumulation, mainly in the 0–5 cm soil layer, at rates related to the addition of crop residues, which were increased by legume cover crops and N fertilization. Considering the reference treatment, the SOC accumulation rates in the 0–20 cm layer varied from 0.09 to 0.34 Mg ha−1 year−1 in CT and from 0.19 to 0.65 Mg ha−1 year−1 in NT. However, the SOC accumulation rates peaked during the first years (5th to 9th) after the adoption of the management practices and decreased exponentially over time, indicating that conservation soil management was a short-term strategy for atmospheric C mitigation. On the other hand, when the CE costs of tillage operations were taken into account, the benefits of NT to C mitigation compared to CT were enhanced. When CE costs related to N-based fertilizers were taken into account, the increases in SOC accumulation due to N did not necessarily improve atmospheric C mitigation, although this does not diminish the agricultural and economic importance of inorganic N fertilization.

The one-compartment C model C t = C 0 e − k 2 t + k 1 A / k 2 ( 1 − e − k 2 t ) is being long used to simulate soil organic C (SOC) stocks. C t is the SOC stock at the time t ; C 0 , the initial SOC stock; k 2 , the annual rate of SOC loss (mainly mineralization and erosion); k 1 , the annual rate to which the added C is incorporated into SOC; and A , the annual C addition. The component C 0 e − k 2 t expresses the decay of C 0 and, for a time t , corresponds to the remains of C 0 ( C 0 remains ). The component k 1 A / k 2 ( 1 − e − k 2 t ) refers, at time t , to the stock of SOC derived from C crops ( C crop ). We herein propose a simple method to estimate k 1 and k 2 coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions ( A ) and SOC stocks. We estimated k 1 and k 2 for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N -urea rates (0 kg N ha −1 −0 N and 180 kg N ha −1 −180 N) in a long-term experiment established in a subtropical Acrisol with C 0 = 32.55 Mg C ha −1 in the 0–17.5 cm layer. A linear equation ( C t = a + bA ) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model C t = C 0 e − k 2 t + k 1 A / k 2 ( 1 − e − k 2 t ) , so that a = C 0 e − k 2 t and b A = k 1 A / k 2 ( 1 − e − k 2 t ) . Such equivalences thus allow the calculation of k 1 and k 2 . NT soil had a lower rate of C loss ( k 2 = 0.019 year −1 ) than CT soil ( k 2 = 0.040 year −1 ), while k 1 was not affected by tillage (0.148 year −1 under CT and 0.146 year −1 under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state ( C e ) in the 0–17.5 cm layer ranged from 15.65 Mg ha −1 in CT O/M 0 N to 60.17 Mg ha −1 in NT OV/MC 180 N. The SOC half-life ( t 1/2 = ln 2/ k 2 ) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C 0 remais ), while increments in C crop are imparted mainly by crop additions.

Nitrogen-fixing crops and N fertilisation increase soil acidification, but few studies have attempted to evaluate the capacity of soil organic matter to alleviate the Al toxicity in acid no-tilled soils. This study was carried out in a 21-year-old experiment aiming to evaluate the effect of crop systems [fallow/maize (Zea mays L.), F/M; oat (Avena strigosa Schreb)/maize, O/M; oat + vetch (Vigna sativa L.)/maize, O+V/M; lablab (Dolichos lablab) + maize, L+M; and pigeon pea (Cajanus cajan L. Millsp.) + maize, P+M] and mineral N fertilisation (0 and 149 kg/ha.year) on chemical attributes and Al speciation in the 0–0.05 and 0.05–0.10 m layers of a no-tilled Paleudult of Southern Brazil. The original soil pH (5.8) decreased in all crop systems, declining 0.37–1.52 units in 21 years without re-liming. This decrease was larger in subsoil layers and, in general, was exacerbated by legume-based crop systems and by N fertilisation. The drop in soil pH increased Al content in solid phase (range 0.07–1.85 cmolc/kg) and in soil solution (range 0.01–0.06 mmol/L), and decreased base saturation on cation exchange capacity (range 12.5–61.2%). However, the Al3+ activity in the soil solution (1.03×10−7–9.3×10−8) was kept below threshold values of toxicity to maize roots, primarily due to the formation of organometallic complexes at low pH, which was estimated as up to 90% of the total Al in solution, but also due to the increased ionic strength in this no-till soil (0.0026–0.0104). Our results highlight that, although legume cover crops and N fertilisation can accelerate soil acidification, Al toxicity is offset by increased organic matter in no-till subtropical soils.

Cropping systems and N fertilisation affect soil acidification mainly due to the removal of alkaline plant material from the field and nitrate leaching. The study evaluated the acidification of a subtropical soil under no till cropping systems with different C and N addition rates for 19 years. The contributions of leguminous and non-leguminous crops (fallow/maize, black oat/maize, black oat + vetch/maize, black oat + vetch/maize + cowpea, lablab + maize, pigeon pea + maize, and digitaria) and mineral N fertiliser (0 and 180 kg N/ha.year as urea) to total acidification were estimated. Cropping systems and N fertilisation significantly affected soil pH, which ranged from 4.3 to 5.1. The presence of leguminous species and mineral N promoted greater decreases in soil pH and net soil acidification, which resulted in increases in exchangeable Al content and Al saturation. Black oat + vetch/maize with N fertilisation promoted the highest soil net acidification rate (2.65 kmol H+/ha.year), while digitaria had the lowest (1.07 kmol H+/ha.year). Leguminous species and N fertilisation increased soil acidification through changes in the C cycle associated with the removal of alkaline plant material by grains. Leguminous-based cropping systems promoted higher maize yields than those comprising essentially gramineous species, indicating an opportunity for a reduction in N fertiliser rates. With N application, however, maize yield did not differ among cropping systems, despite differences in soil pH and exchangeable Al.