Land use effects on labile N extracted with permanganate and the nitrogen management index in the Cerrado region of Brazil

 The effect of land use on the availability of nitrogen (N) was studied by separating total N into a labile and a stable fraction by oxidation and extraction of labile N with KMnO₄. The nitrogen management index (NMI) was calculated following the method of Blair et al. (1995) for the carbon management index. In all systems, labile N released by KMnO₄ was a better indicator of N availability than total and stable N. The NMI was a good indicator of N availability but gave no information about the total amount of N. In land use system analysis, total N and labile N can be used together as a simple and rapid way to evaluate the N status of the soil. Legume-based pastures specifically increased the amount of labile N. Although soybeans had a dominant role in the continuous cropping systems studied, the total N contents of these systems were lower than those of the natural Cerrado. The availability of N under legume-based pastures and legume-based pasture-crop rotations was higher than under the natural Cerrado and the continuous cropping systems. Received: 11 June 1997     

Earthworm communities and soil properties in shaded coffee plantations with and without application of glyphosate

In central Veracruz, Mexico, many coffee plantations are managed using agrochemicals for weed control, with glyphosate-based herbicides (GBH) the most commonly used. To date, however, no studies in this region have characterized the soil biological and physicochemical properties in coffee plantations under such glyphosate application. In this study, earthworms were used as bioindicator organisms by measuring differences in the earthworm community in plots within shaded coffee plantations, with and without repeated applications of glyphosate. Differences in earthworm-induced soil processes, such as water infiltration rates, potential net carbon mineralization rates and soil physicochemical properties were also evaluated. Eight plots were selected in shaded coffee plantations; four had received regular applications of GBH over the preceding 22 years, while the other four had received no herbicides over the preceding 7 years. The earthworm species found in plots with no GBH treatment were Pontoscolex corethrurus (99%) and Amynthas corticis (1%), while A. corticis was absent in plots that had been treated with GBH. Significant differences (P < 0.01) in earthworm density (168 ± 16 and 353 ± 37 ind m⁻²) and biomass (22.7 ± 1.1 and 45.4 ± 6.9 g m⁻²) were observed in soils with and without GBH, respectively. No significant difference (P = 0.08) was observed in the water infiltration rate (2 × 10⁻⁴ ± 4 × 10⁻⁵ and 4 × 10⁻⁴ ± 1 × 10⁻⁴ cm s⁻¹ with and without GBH, respectively). Soil carbon flow was greater in plots with GBH (76 ± 7 μg dry soil⁻¹ d⁻¹) than in those without GBH (62 ± 1 μg dry soil⁻¹ d⁻¹, P < 0.005). Significant differences (P < 0.05) were found in pH and in the clay, silt and Ca content of the soil. Our findings indicated reduced species number, density and biomass of earthworms, and increased net carbon mineralization rate in plots with GBH. The plots managed with glyphosate presented a negative effect on the earthworm parameters measured, and we conclude that the earthworms therefore acted as indicators of perturbation. It is also possible that this effect could be due to factors unrelated to the glyphosate that were not considered in this study, such as chemical fertilization or legume litter spatial variability, among others.     

Medium-term effects of corn biochar addition on soil biota activities and functions in a temperate soil cropped to corn

Biochar addition to soil has been generally associated with crop yield increases observed in some soils, and increased nutrient availability is one of the mechanisms proposed. Any impact of biochar on soil organisms can potentially translate to changes in nutrient availability and crop productivity, possibly explaining some of the beneficial and detrimental yield effects reported in literature. Therefore, the main aim of this study was to assess the medium-term impact of biochar addition on microbial and faunal activities in a temperate soil cropped to corn and the consequences for their main functions, litter decomposition and mineralization. Biochar was added to a corn field at rates of 0, 3, 12, 30 tons ha⁻¹ three years prior to this study, in comparison to an annual application of 1 t ha⁻¹.Biochar application increased microbial abundance, which nearly doubled at the highest addition rate, while mesofauna activity, and litter decomposition facilitated by mesofauna were not increased significantly but were positively influenced by biochar addition when these responses were modeled, and in the last case directly and positively associated to the higher microbial abundance. In addition, in short-term laboratory experiments after the addition of litter, biochar presence increased NO₂ + NO₃ mineralization, and decreased that of SO₄ and Cl. However, those nutrient effects were not shown to be of concern at the field scale, where only some significant increases in SOC, pH, Cl and PO₄ were observed.Therefore, no negative impacts in the soil biota activities and functions assessed were observed for the tested alkaline biochar after three years of the application, although this trend needs to be verified for other soil and biochar types.     

Organic nitrogen cycling microbial communities are abundant in a dry Australian agricultural soil

Some microbial nitrogen (N) cycling processes continue under low soil moisture levels in drought-adapted ecosystems. These processes are of particular importance in winter cropping systems, where N availability during autumn sowing informs fertilizer practices and impacts crop productivity. We evaluated the organic and inorganic N-cycling communities in a key cropping soil (Vertosol), using a controlled-environment incubation study that was designed to model the autumn break in south Australian grain growing regions. Soils from wheat, lucerne, and green manure (disced-in vetch) rotations of the Sustainable Cropping Rotations in Mediterranean Environments trial (Victoria, Australia) were collected during the summer when soil moisture was low. Microbial community structure and functional capacity were measured both before and after wetting (21, 49, and 77 days post-wetting) using terminal restriction fragment length polymorphism measures of bacterial and fungal communities, and quantitative PCR of nitrogen cycling genes. Quantified genes included those associated with organic matter decomposition (laccase, cellobiohydrolase), mineralization of N from organic matter (peptidases) and nitrification (bacterial and archaeal ammonia monooxygenase and nitrite oxidoreductase). In general, the N cycling functional capacity decreased with soil wetting, and there was an apparent shift from organic-N cycling dominance to autotrophic mineral-N cycling dominance. Soil nitrate levels were best predicted by laccase and neutral peptidase genes under drought conditions, but by neutral peptidase and bacterial ammonia monooxygenase genes under moist conditions. Rotation history affected both the structural and functional resilience of the soil microbial communities to changing soil moisture. Discing in green manure (vetch) residues promoted a resilient microbial community, with a high organic-N cycling capacity in dry soils. Although this was a small-scale microcosm study, our results suggest that management strategies could be developed to control microbial organic-N processing during the summer fallow period and thus improve crop-available N levels at sowing.     

Soil characteristics determine soil carbon and nitrogen availability during leaf litter decomposition regardless of litter quality

Climate and litter quality have been identified as major drivers of litter decomposition, but our knowledge of how soil characteristics (e.g. microbial community and chemical properties) determine carbon (C) and nitrogen (N) availability derived from the decomposition of litter of different qualities is still scarce. We conducted a microcosm experiment to evaluate how soils with contrasting microbial communities and soil properties (denoted Soils A and B hereafter, where Soil B has higher bacterial and fungal abundance, fungal:bacterial ratio, and organic C than Soil A) determine the availability of soil C (carbohydrates, proteins, amino acids and phenols) and N (dissolved organic and inorganic N, microbial biomass N and available N) during the decomposition of litter of contrasting quality (C:N ratios ranging from 20 to 102). We also evaluated the relative importance of soil characteristics and litter quality as drivers of C and N inputs to the soil during this process. Overall, higher soil C and N availability after litter decomposition was found in Soil B than in Soil A. Soil characteristics had a higher positive effect on soil C and N contents than litter quality during litter decomposition. We also found that changes in N availability and organic matter quality registered after litter decomposition, linked to different soil characteristics, were able to promote dissimilarities in the potential mineralization rates. In conclusion, our study provides evidence that soil characteristics (e.g. microbial communities and chemical properties) can be more important than litter quality in determining soil C and equally important for N availability during the decomposition of leaf litter.     

Long-term conservation tillage results in a more balanced soil microbiological activity and higher nutrient supply capacity

Soil health depletion due to intensive tillage operations is a global issue in the agricultural sector. Conservation tillage (CT) which involves non-inversion tillage and leaving ∼30% of the soil surface covered with crop residues, is a strategy designed to enhance soil health. However, no comprehensive study to investigate the long-term effect of CT on soil biological activity and the soil nutrient supply has yet been widely carried out. Biological and chemical soil properties were assessed at depths 0–5, 10–15, and 20–25 cm depths after 18 years of CT and conventional tillage practice (PT). Various stages in the vegetative growth of maize were investigated in 2021 in Hungary. The findings indicated that tillage intensity, soil depth, and growth stages all significantly influenced soil enzyme activities and the concentration of soil nutrients. Less soil disturbance resulted in a significantly larger concentration of soil carbon parameters (total organic carbon and labile carbon) in CT plots, where the activity of β-glucosidase and dehydrogenase (DHA) in the upper soil layer increased significantly (0.7–2.6 and 2.6–4.7 times, respectively) compared to PT. The high amount of organic matter and the greater resistance to erosion observed in CT also contributed to the higher concentration of available nutrients (NH₄, NO₃, Ca, K) and total P in the surface soil layer. Phosphatase activity was highest in the mid-stage of vegetative growth and was positively correlated to the total P concentration. The alterations in soil water content were clearly negatively correlated with the change in DHA and phosphatase activity. Overall, due to the more balanced environmental conditions, the decomposition of organic substances was more balanced and slower in CT than in PT. This implied that the mobilization of nutrients in the soil was more balanced as well, and that the nutrients were released gradually. The enhancement of the soil nutrient-supplying capacity achieved by means of long-term conservation tillage provides a promising strategy for sustainable nutrient management.     

Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m×1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater (P?0.01) in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased (P?0.10) by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.     

乙炔抑制方式对潮土硝化和矿化作用的影响

在土壤最大持水量60%和温度28?C的实验室培养条件下,研究了乙炔的不同抑制方式(短时前期暴露/连续灌注法)对潮土硝化、矿化作用的影响。结果显示,连续14天灌注10 ml/L或100 ml/L浓度乙炔完全抑制了供试潮土的硝化作用,而将土样短时前期暴露于乙炔12 h后驱散乙炔,仅能保持48 h的抑制效果,驱散乙炔后第3天土样的硝化作用开始恢复,培养结束时土样的硝化率仍可达到99%。本研究的结果还显示,培养结束时,加入乙炔的4个施N处理其净矿化量为负值,乙炔连续抑制方式下土样净矿化量低于乙炔短时前期暴露方式下土壤的净矿化量。因此,采用乙炔抑制技术进行研究时应当考察所采集的供试土壤在乙炔抑制方式下的硝化活性恢复速率和矿化过程,以保证相关试验方法设计的合理性。对培养期较长的试验,采用连续灌注乙炔,并将通风时间控制在硝化活性恢复点以前的方式是区分硝化类型较为适用的实验室方法。     

Effect of timing of application of municipal solid waste compost on N availability for crops in central Spain

To calculate the correct nitrogen fertilizer rate for crops and the possibility of using municipal solid waste (MSW) compost as an organic amendment, nitrogen mineralization rates were studied by laboratory incubation and field measurements in a soil in central Spain. Nitrogen mineralization rates were studied in a 250-day laboratory soil incubation with two treatments: with and without compost, incubated at 28°C and a moisture content of 70% of field capacity. Three phases are described: (1) no increase in the mineral nitrogen content, (2) a linear increase in the mineral N fraction and, finally, (3) a linear, parallel increase in both mineral N and easily mineralizable organic N fractions. Incubation data were fitted to three different equations. The exponential model proposed by Stanford and Smith (1972) was selected to predict field N mineralization rates. The field experiment was performed using a crop of maize with three treatments: compost applied in February (before sowing), compost applied during sowing and a control (without compost application): sampling was carried out over 14 months. Soil water content was measured periodically. Soil with compost applied in February showed 1.9 and 1.4 times more available nitrogen than soil without compost and compost at sowing, respectively, for the month of maximum accumulation. These results suggest that compost amendments must be applied before sowing. Compost applications were shown to supply the available nitrogen for spring crops. A simulation model showed satisfactory agreement with field data, after correction for soil temperature and water content. Received: 22 July 1996     

Carbon mineralization kinetics as influenced by soil properties

In a short-term laboratory study C mineralization potentials were determined on soil samples obtained from some representative agricultural soils in Tuscany, Italy. All the kinetic models tested to describe the mineralization process provided a good fit to the experimental data. A modified first-order model best described C mineralization in the soil. Both potentially mineralizable C and the mineralization rate (k) varied considerably among soils, reflecting the differences in soil properties. Potentially mineralizable C was positively related to C evolved as CO₂ and to the exchange capacity. Normalized values (potentially mineralizable C divided by organic C), representing on average about 2% of the total soil C, was positively correlated to soil pH and negatively to the soil C pool, the soil N pool, and total microbial activity. Values for k ranged between 0.050 and 0.104 day^-1, being higher in fine-textured soils and in soils with a large free Fe content. A low C:N ratio was indicative of a high k value. Turnover times for mineralized C were relatively rapid, ranging from 10 to 20 days.