Nele Verhulst

Greenhouse gas emissions under conservation agriculture compared to traditional cultivation of maize in the central highlands of Mexico

In 1991, the International Maize and Wheat Improvement Center (CIMMYT) started a field experiment in the rain fed Mexican highlands to investigate conservation agriculture (CA) as a sustainable alternative for conventional maize production practices (CT). CT techniques, characterized by deep tillage, monoculture and crop residue removal, have deteriorated soil fertility and reduced yields. CA, which combines minimum tillage, crop rotations and residue retention, restores soil fertility and increases yields. Soil organic matter increases in CA compared to CT, but increases in greenhouse gas emissions (GHG) in CA might offset the gains obtained to mitigate global warming. Therefore, CO(2), CH(4) and N(2)O emissions, soil temperature, C and water content were monitored in CA and CT treatments in 2010-2011. The cumulative GHG emitted were similar for CA and CT in both years, but the C content in the 0-60 cm layer was higher in CA (117.7 Mg C ha(-1)) than in CT (69.7 Mg C ha(-1)). The net global warming potential (GWP) of CA (considering soil C sequestration, GHG emissions, fuel use, and fertilizer and seeds production) was -7729 kg CO(2) ha(-1) y(-1) in 2008-2009 and -7892 kg CO(2) ha(-1) y(-1) in 2010-2011, whereas that of CT was 1327 and 1156 kg CO(2) ha(-1) y(-1). It was found that the contribution of CA to GWP was small compared to that of CT.

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

Background and aimsConservation 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.MethodsThree 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 5–10xa0cm soil layers and separated in micro-aggregates (< 0.25xa0mm), small macro-aggregates (0.25 to 1xa0mm) and large macro-aggregates (1 to 8xa0mm). The carbon content of each aggregate fraction was determined.ResultsZero tillage combined with crop rotation and crop residues retention resulted in a higher proportion of macro-aggregates. In the 0–5xa0cm 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) (1.7xa0mm). The plots with CT had the largest proportion of micro-aggregates (27%). In the 5–10xa0cm layer, plots with residue retention in both CT and ZT (maize 1xa0mm and wheat 1.5xa0mm) or with monoculture of wheat in plots under ZT without residues (1.4xa0mm) had the greatest MWD. The 0–10xa0cm soil layer had a greater proportion of small macroaggregates compared to large macro-aggregates and micro-aggregates. In the 0–10xa0cm 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.ConclusionReduction 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).

Bacterial indicator taxa in soils under different long-term agricultural management.

In this study, the species indicator test was used to identify key bacterial taxa affected by changes in the soil environment as a result of conservation agriculture or conventional practices.

The Bacterial Community Structure and Dynamics of Carbon and Nitrogen when Maize (Zea mays L.) and Its Neutral Detergent Fibre Were Added to Soil from Zimbabwe with Contrasting Management Practices.

Water infiltration, soil carbon content, aggregate stability and yields increased in conservation agriculture practices compared to conventionally ploughed control treatments at the Henderson research station near Mazowe (Zimbabwe). How these changes in soil characteristics affect the bacterial community structure and the bacteria involved in the degradation of applied organic material remains unanswered. Soil was sampled from three agricultural systems at Henderson, i.e. (1) conventional mouldboard ploughing with continuous maize (conventional tillage), (2) direct seeding with a Fitarelli jab planter and continuous maize (direct seeding with continuous maize) and (3) direct seeding with a Fitarelli jab planter with rotation of maize sunn hemp (direct seeding with crop rotation). Soil was amended with young maize plants or their neutral detergent fibre (NDF) and incubated aerobically for 56xa0days, while C and N mineralization and the bacterial community structure were monitored. Bacillus (Bacillales), Micrococcaceae (Actinomycetales) and phylotypes belonging to the Pseudomonadales were first degraders of the applied maize plants. At day 3, Streptomyces (Actinomycetales), Chitinophagaceae ([Saprospirales]) and Dyella (Xanthomonadales) participated in the degradation of the applied maize and at day 7 Oxalobacteraceae (Burkholderiales). Phylotypes belonging to Halomonas (Oceanospirillales) were the first degraders of NDF and were replaced by Phenylobacterium (Caulobacterales) and phylotypes belonging to Pseudomonadales at day 3. Afterwards, similar bacterial groups were favoured by application of NDF as they were by the application of maize plants, but there were also clear differences. Phylotypes belonging to the Micrococcaceae and Bacillus did not participate in the degradation of NDF or its metabolic products, while phylotypes belonging to the Acidobacteriaceae participated in the degradation of NDF but not in that of maize plants. It was found that agricultural practices had a limited effect on the bacterial community structure, but application of organic material altered it substantially.

Fate of atrazine in a soil under different agronomic management practices

Agricultural management affects the movement of atrazine in soil and leaching to groundwater. The objective of this study was to determine atrazine adsorption in a soil after 20 years of atrazine application under agronomic management practices differing in tillage practice (conventional and zero tillage), residue management (with and without residue retention) and crop rotation (wheat-maize rotation and maize monoculture). Atrazine sorption was determined using batch and column experiments. In the batch experiment, the highest distribution coefficient Kd (1.1 L kg−1) at 0–10 cm soil depth was observed under zero tillage, crop rotation and residue retention (conservation agriculture). The key factor in adsorption was soil organic matter content and type. This was confirmed in the column experiment, in which the highest Kd values were observed in treatments with residue retention, under either zero or conventional tillage (0.81 and 0.68 L kg−1, respectively). Under zero tillage, the fact that there was no soil movement helped to increase the Kd. The increased soil organic matter content with conservation agriculture may be more important than preferential flow due to higher pore connectivity in the same system. The soils capacity to adsorb 2-hydroxyatrazine (HA), an important atrazine metabolite, was more important than its capacity to adsorb atrazine, and was similar under all four management practices (Kd ranged from 30 to 40 L kg−1). The HA adsorption was attributed to the type and amount of clay in the soil, which is unaffected by agronomic management. Soils under conservation agriculture had higher atrazine retention potential than soils under conventional tillage, the system that predominates in the study area.

Nitrogen fertilizer placement and timing affects bread wheat (Triticum aestivum) quality and yield in an irrigated bed planting system

Searching for sustainable cropping systems has often focused on the optimization of system sustainability, yield or ideally both. It is important to also include wheat quality characteristics like grain protein concentration (GPC) and bread-making quality (loaf volume). The effect of placement and timing of nitrogen (N) fertilizer on performance and quality of the bread wheat cultivar Navojoa, was tested in two tillage systems under furrow irrigation: conventionally tilled beds (CTB; new beds formed after disc ploughing and residue incorporation) and permanent beds (PB; only furrows reshaped and residues retained on the surface). N fertilizer (120xa0kgxa0Nxa0ha−1 as urea) was broadcast, applied in furrows or disk-banded on top of beds. The timing treatments were all fertilizer applied before planting or split between pre-planting and first node. Permanent beds had the highest yield and grain quality was highest with split bed and furrow application. Averaged over four years, N use efficiency and GPC were significantly higher in CTB (24.5xa0kgxa0kg−1 and 10.1xa0%, respectively) than in PB (22.2xa0kgxa0kg−1 and 9.9xa0%, respectively) indicating the necessity in PB for optimal N management to obtain stable yields and acceptable grain quality. A reduced number of cold hours and solar radiation negatively affected grain yield. Broadcast fertilizer application reduced grain quality and N use efficiency. N fertilizer management in furrow-irrigated wheat cropping systems should combine splitting the N dose and disking it on the bed pre-planting and in the furrow later in the season, depending on the crop needs at the application time.

Bacterial colonization of a fumigated alkaline saline soil

AbstractnAfter chloroform fumigating an arable soil, the relative abundance of phylotypes belonging to only two phyla (Actinobacteria and Firmicutes) and two orders [Actinomycetales and Bacillales (mostly Bacillus)] increased in a subsequent aerobic incubation, while it decreased for a wide range of bacterial groups. It remained to be seen if similar bacterial groups were affected when an extreme alkaline saline soil was fumigated. Soil with electrolytic conductivity between 139 and 157xa0dSxa0m−1, and pH 10.0 and 10.3 was fumigated and the bacterial community structure determined after 0, 1, 5 and 10xa0days by analysis of the 16S rRNA gene, while an unfumigated soil served as control. The relative abundance of the Firmicutes increased in the fumigated soil (52.8xa0%) compared to the unfumigated soil (34.2xa0%), while that of the Bacteroidetes decreased from 16.2xa0% in the unfumigated soil to 8.8xa0% in the fumigated soil. Fumigation increased the relative abundance of the genus Bacillus from 14.7xa0% in the unfumigated soil to 25.7xa0%. It was found that phylotypes belonging to the Firmicutes, mostly of the genus Bacillus, were dominant in colonizing the fumigated alkaline saline as found in the arable soil, while the relative abundance of a wide range of bacterial groups decreased.

Greenhouse Gas Emissions from Nontilled, Permanent Raised, and Conventionally Tilled Beds in the Central Highlands of Mexico

Organic matter content increases in soil with no-tilled permanent raised beds (PBs) compared with soil with conventionally tilled beds (CBs), and this might affect greenhouse gas (GHG) emissions. Greenhouse gas (CO2, N2O, and CH4) emissions were measured from PBs, from which crop residue was either removed or retained and from CBs where crop residue was retained. The CO2 emission was not affected by tillage, but CH4 and N2O emissions were lower in PBs when residue was retained than in CBs. Removing crop residue from PBs reduced CO2 emissions compared with when it was retained, but it had no effect on N2O and CH4 emissions. The global warming potential (GWP) of GHG emissions was higher in CBs (801 kg CO2/ha/year) than in PBs (517 kg CO2/ha/year) with crop-residue retention, but more C was sequestered in the 0–60 cm soil layer in PBs (83.4 Mg C/ha) than in CBs (79.2 Mg C/ha). Crop-residue removal in PBs had little effect on the GWP of GHG compared with PBs with crop residue retained, but less C was sequestered in the latter (63.1 Mg C/ha). Net GWP (considering soil C sequestration, GHG emissions, fuel used, glyphosate application, fertilizer and seed production) was lower in CBs with crop-residue retention (1062 kg CO2/ha/year) than in PBs with crop-residue removal (6,120 kg CO2/ha/year), but it was larger than in PBs with crop-residue retention (−681 kg CO2/ha/year). We found that reduced tillage when beds were made permanent and crop-residue retention greatly reduced net GWP compared with when beds were tilled and remade each year.We found that retention of crop residue in PBs increased the emission of CO2 compared with where it was removed, but tillage did not affect fluxes of CO2. Emission of CH4 and N2O was larger from CBs than from PBs, but crop-residue management in PBs had no significant effect on fluxes of CH4 and N2O. Concentrations of mineral N were larger in CBs than in PBs, whereas the removal of crop residue from PBs increased mineral N concentration. Soil temperature was higher in CBs than in PBs and in PBs with crop residue retained compared with where it was removed. Soil water was better preserved in PBs than in CBs and in PBs where residue was retained than where it was removed. The higher water content in the PB compared with the CB will favor plant growth during dry spells. However, retaining crop residues in PBs will require sufficient application of inorganic N, as mineral N in soil is lower in PBs than in CBs or PBs with crop residue removed. Limited N availability in PBs with crop residue retained might reduce yields as poor farmers in the central highlands of Mexico apply little or no N fertilizer. Reduced tillage on PBs and crop-residue retention strongly reduced the net GWP of the system compared with the case when beds were remade each year. PBs with residue retention reduced net GWP by 50% compared with CBs with residue retention, but the removal of residues from the PBs more than doubled it.