High Quality Residues Research Articles

Plant organ rather than cover crop species determines residue incorporation into SOC pools

The implementation of cover crops has emerged as a promising approach to improve soil organic carbon (SOC) stocks, with particular emphasis on the perceived higher carbon use efficiency displayed by high-quality residues such as from leguminous plants. In this study, we explored how different cover crop residues, specifically from a legume and a grass cover crop, affects SOC formation and its distribution across various soil carbon pools. Over a 7-month period, we incubated 14C-labeled winter rye and hairy vetch residues in microcosms containing soils of varying soil fertility levels from a long-term field trial. We tracked the fate of carbon into free and occluded particulate organic matter (fPOM, oPOM), mineral-associated organic matter (MAOM), and carbon deposited outside the detritusphere.Despite notable differences in C:N ratio, chemical composition, and turnover rate, similar SOC formation efficiency between vetch and rye within each plant organ (shoots and roots) was observed. Interestingly, the plant organ appeared to exert a greater influence on the fate of cover crop carbon than whether the crop was leguminous or non-leguminous. This phenomenon seemed to be closely related to the lignin content.At medium soil fertility, we found that the largest proportion of cover crop residue C remained as MAOM (20% for shoots, 15–18% for roots), followed by fPOM (5–6% for shoots, 10–12% for roots) and oPOM (2.7–3.0% for shoots, 1.5–1.6% for roots). Notably, fPOM and oPOM exhibited opposite responses to residue quality, indicating functional distinctions between these often-pooled POM pools.Soil fertility exerted minimal influence on overall respiration rate patterns or SOC formation, although it did affect oPOM formation efficiency, likely due to differences in soil aggregation.In conclusion, our findings challenge the assumption regarding the superiority of N rich leguminous cover crop residues for enhancing SOC accrual in C pools believed to have longer persistence.

Aromatization of residue caused by group aggregation of hyperbranched ester macromolecule capped with benzene rings improved flame retardancy of epoxy resin

To investigate the effect of the distribution state of phosphophenanthrene groups within the branched structure on the flame retardant efficiency, the hyperbranched ester macromolecules Bz-DQMC-n (n = 1, 2, 3) capped with benzene rings were synthesized. The aggregation effect of phosphaphenanthrene groups in Bz-DQMC-n enhanced the flame retardancy of epoxy resins (EPs). At the same phosphorus contents of EPs, the Bz-DQMC-n/EP passed the V-0 rating, except for 2-(6-oxid-6H-dibenzo[c,e][1,2]oxaphosphorin-6-yl)-1,4-benzenediol (DOPO-HQ)/EP without aggregated phosphaphenanthrene groups, which only reached the V-1 rating in UL 94 test. The Bz-DQMC-2 showed a better flame inhibition effect. The limiting oxygen index (LOI) value of the Bz-DQMC-2/EP reached to 39.3%, and the peak heat release rate (pk-HRR) value was 563 kW/m2, which decreased by 62.5% compared to neat EP. In addition, the total heat release (THR), and average effective heat of combustion (av-EHC) values of EP with Bz-DQMC-2 were also lower than that with Bz-DQMC-1 and Bz-DQMC-3 in the cone calorimetry test. The mechanism demonstrated that the aggregated phosphophenanthrene groups and benzene rings in Bz-DQMC-2 jointly promoted the formation of high-quality residue with more aromatic structures. Meanwhile, the concentrated release feature caused by group aggregation slightly enhanced the quenching effect in the gas phase. The aggregation effect of phosphophenanthrene groups in specific structures provides guidance for the development of high-efficiency DOPO-based flame retardants.

Influence of agricultural wastes on larval growth phases of the black soldier fly, Hermetia illucens (Diptera: Stratiomyidae): An integrated approach

Insects are an effective tool for converting nutrients in agricultural by-products into protein-rich biomass and compost. Black soldier fly (BSF) (Hermetia illucens) larvae are currently one of the insect species widely used as a protein source in aquafeed globally. Although much effort has been spent on the use of BSF in aquafeed, there is not much information on the biology of the insect, especially with the morphology of the BSF. This study aimed to evaluate the influence of various organic wastes, such as fruit wastes (FW) and vegetable wastes (VW), on different growth phases of BSF larva (BSFL), using morphometric and scanning electron microscopic examinations, and the composition of the compost produced, as well as a method for upscaling of larval production of BSFL. Faster growth was observed in BSFL fed with VW substrate (40 days) compared to the FW (46 days). Based on the morphometric measurements such as length, larval head length, total length etc., five larval stages, prepupal and pupal stage of BSFL were differentiated and described. In addition, SEM imaging of BSF mouth parts found that the mouth morphology of the BSF larvae and prepupal stage differed, and the BSF prepupa had reduced mouthparts. Also, the mandibular-maxillary complex was well developed than the BSF prepupa. BSFL larvae have proven to convert fruit and vegetable waste into high-quality residue fertilizer for the soil. The BSF compost showed optimum nitrogen, phosphorous, potassium, calcium and sulphur content. This research establishes a baseline knowledge and guidance on the BSF-rearing facilities.

Pine sawdust modification using Fenton oxidation for enhanced production of high-yield lignin-containing microfibrillated cellulose

Sawdust is an abundant high-quality residue from sawmills, representing 20–30 % of sawn products by volume. In this study, the chemical pre-treatment of pine sawdust with Fenton’s reagent, formed from hydrogen peroxide and iron catalyst under moderately acidic conditions, was found to intensify the microfibrillation process in terms of energy consumption and improve the grade of the high-yield lignin-containing microfibrillated cellulose (LMFC) produced.With a minor yield loss of 5.5 wt.%, Fenton pre-treatment increased the microfibrillation rate and bonding potential of LMFC, indicating that the ultrastructure of the lignocellulose cell walls had been modified. Linear dependency between the growth of specific surface area and energy consumption was seen, i.e. microfibrillation followed Rittinger’s law of comminution. In comparison with the reference without any pretreatment,the total grinding energy consumption to a particle size of 14 µm was about 30 % lower (10.7 vs. 15 MWh/t) while the tensile strength and stiffness of LMFC films were 50 % (100 vs. 66 MPa) and 35 % higher (6.6 vs. 4.9 GPa), respectively. The advantageous effects of Fenton chemistry were assumed to originate from the cleavage of lignin-carbohydrate bonds, mainly between lignin and hemicelluloses. This phenomenon was supported by the substantially increased solubility of polysaccharides in dilute alkali.The calculated manufacturing costs of LMFCs (using the above-mentioned specifications) was € 850/t, of which the raw material, chemical and electricity costs accounted for 10 %, 2 % and 88 %, respectively. Without any chemical pre-treatment, manufacturing costs were € 1100/t of which raw material accounted for 7 % and electricity 93 %.

Intensive crop rotations and residue quality increase soil phosphorus lability under long-term no-till in tropical soils

This study assessed soil Phosphorus (P) fractions and lability according to crops in rotation and crop residue P inputs under long-term no-till (NT) in weathered tropical soils. Two experiments were established in split-plot designs with fall-winter as the main plot and spring as the subplots. In one experiment, sunflower and triticale were grown in Typic Rhodudalf (TR) soil in fall-winter; in the other experiment, monocropped and intercropped ruzigrass and grain sorghum were grown in Rhodic Hapludox (RH) soil in fall-winter. In both experiments, sunn hemp, forage sorghum, and pearl millet were grown in the spring; an additional fallow treatment was established in spring in TR. Soybean was grown every year in the summer in both experiments. After 12 (TR) and 9 (RH) years, the crop residue quality (e.g. N content and lignin-N ratio) and P inputs of the various crops were assessed. In addition, soil P fractions were determined according to the Hedley procedure. High-quality residue enhanced P recycling by soybean under crop rotation with sunn hemp in both experiments, sunflower in TR, and ruzigrass in RH. Sunn hemp increased soil labile P by 13% compared with pearl millet in TR and by 40% compared with forage sorghum and pearl millet in RH. Forage sorghum in spring enhanced P mod-labile in TR, effect also observed for monocropped grain sorghum and ruzigrass in fall-winter in RH compared to both intercropped. In general, crop rotation under NT improved soil P lability, particularly the biological P pool, suggesting positive impacts from crop residue quality. The results of this study can be used to guide P fertilization strategies for NT cropping systems in tropical soils.

Organic phosphorus forms in a tropical sandy soil after application of organic residues of different quality

The addition of organic materials can improve soil fertility and phosphorus availability in agricultural soils. However, knowledge of organic P (Po) forms in soils with the incorporation of different quality residues is limited. This study investigated Po forms and determined the relationship between Po and soil properties in tropical sandy soils after application of local organic residues of different qualities, including groundnut stover (GN), tamarind leaf litter (TM), dipterocarp leaf litter (DP), and rice straw (RS). The Po forms were determined using a sequential extraction procedure and 31P nuclear magnetic resonance (NMR) spectroscopy. Addition of residues, regardless of quality, had little effect on total Po accumulation, but it affected soil Po forms. Labile Po was a dominant form after incorporating high-quality residue (high nitrogen, low lignin, and low polyphenols) (e.g., GN), whereas nonlabile humic-Po and residual Po were dominant forms in soils treated with lower-quality residue with low N, high lignin, and high polyphenols (e.g., TM, DP, and RS). Using 31P NMR spectroscopy, orthophosphate and phosphate monoester (mono-P) were the major forms of inorganic P and organic P in all residue-treated soils, respectively. High-quality residue incorporation increased diester, DNA, and teichoic acid. The results showed that phosphonate occurred in GN soil because of acidic conditions occurring when GN was applied. The Po forms in lower-quality residue additions were dominated by mono-P because these residues had elevated contents of lignin and polyphenols, which has the potential to produce humic substances that form complexes with soil minerals. Furthermore, the nonlabile Po had a positive association with available P in tropical sandy soils.

Broadening farmer options through legume rotational and intercrop diversity in maize-based cropping systems of central Malawi

• Evaluated diverse legume-maize systems in a 4-year, multi-site, on-farm study. • Pigeonpea-maize rotation had the most biomass, ideal for amending infertile soils. • In hot and dry environments, groundnut increased maize yield in a subsequent year. • Pigeonpea biomass was key to high N fixation yet suppressed by intercropping. Smallholder farmers across southern Africa have limited access to adequate quantities of inorganic fertilizers, and this limits crop productivity. Integration of legumes such as groundnut ( Arachis hypogaea L.) and pigeonpea ( Cajanus cajan L.) improve soil fertility and subsequent cereal crop productivity through biological nitrogen fixation (BNF), and high-quality residues. The objective of this four-year, multi-site, on-farm study in Malawi was to evaluate diverse legume systems, quantify crop production, economic viability, and total N accumulation in groundnut and pigeonpea. The study involved six research sites with different agricultural productivity potential based on precipitation, elevation, and temperature; to evaluate crop performance by environment. The six cropping systems evaluated were sole pigeonpea or sole groundnut rotated with maize ( Zea mays L.) (PP-MZ and GN-MZ), groundnut/pigeonpea intercrop rotated with maize (GNPP-MZ), maize/pigeonpea intercrop (MZPP), continuous unfertilized maize (MZ-MZ), and fertilized maize (MZ+F). Total N accumulated by cropping system was in the order GNPP > PP > MZPP > GN at 180, 130, 103 and 87 kg N/ha respectively. In the groundnut/pigeonpea intercrop, despite a reduction of pigeonpea biomass, pigeonpea was still the main driver for N cycling in the systems. The average percentage of N derived from BNF (BNF-N%) from all plant components and across sites was 66% for groundnut and 52% for pigeonpea. Average maize grain yield for response years when rotated with legumes was 4.82, 3.25, and 2.16 Mg/ha for sites with high (Linthipe), medium (Kandeu), and low (Golomoti) agricultural productivity potential. System performance in terms of gross margins ranged from $1145 (pigeonpea-maize rotation) to $1407/ha (groundnut/pigeonpea-maize rotation). The pigeonpea-maize rotation produced the most biomass but had modest economic and agronomic returns during the legume phase. This study highlighted that, for hot and dry environments, groundnut was the most effective species at increasing maize yield in a subsequent year. The groundnut/pigeonpea-maize rotation system met multiple goals including high pigeonpea biomass, profitable groundnut grain, and high maize yields during the rotational year. This system is recommended for ecological intensification, as it provided an economically feasible means to enhance BNF on smallholder farms, while ensuring multiple grain harvest goals in different environmental contexts.

Decomposing cover crops modify root-associated microbiome composition and disease tolerance of cash crop seedlings

The assembly of root-associated microbes during the seedling stage has strong impact on subsequent performance of crops. Major factors influencing this assembly are crop species identity and composition of potential root-colonizing microbes in the bulk soil. The latter can be modified by soil management, such as organic amendments. The incorporation of residues of cover crops before the start of the growing season of cash crops presents an interesting option for steering of root-associated seedling microbiomes as there is a wide range of cover crops species with different properties available for farmers.In a greenhouse study, we examined the effect of soil amendments with milled shoot and root materials of seven cover crop species (niger seed, phacelia, rapeseed, radish, vetch, black oat and buckwheat) on the assembly of root-associated bacteria and fungi of seedlings of four cash crop species (asparagus, carrot, onion and sugar beet). Field-grown cover crops material used for the study was collected at two time points (before and after winter) which had strong impact on plant elemental composition. The soil used for the study was a mixture of sandy arable soils with a history of soil-borne fungal diseases (Fusarium and Rhizoctonia). Within the context of a strong selection of root-associated microbes by cash crop species, we found significant modifying effects by cover crop materials. We show that cover crop elemental composition had a stronger effect than cover crop species identity. High quality residues (with low C/N ratio) caused profound shifts within root-associated Proteobacteria and increases in relative abundance of certain microbial groups such as Bacillaceae and Mortierellomycetes. These changes coincided with differences in establishment and survival of cash crop seedlings. Tolerance of sugar beet seedlings against the fungal pathogen Rhizoctonia solani was correlated with residues causing increases of root-associated Oxalobacteraceae, Bacillaceae and Mortierellaceae. However, the same residues increased Fusarium-induced failure of asparagus seed germination. This indicates that fine-tuning of cover crops amendments for different cash crops is required to realize enhanced functioning of root microbiomes.

Could more efficient utilization of ecosystem services improve soil quality indicators to allow sustainable intensification of Amazonian family farming?

In the Amazonian periphery, there are sources of numerous disservices, including deforestation, loss of wildlife habitat and biodiversity erosion. However, there are great opportunities to adopt appropriate agricultural management practices to take advantage of the benefits of ecosystem services for sustainable agricultural intensification. Thus, the aim of this work was to evaluate the effects of certain ecosystem services provided by combined use of legumes with residue of low- and high-quality on soil quality indicators, nitrogen use efficiency and sustainability of maize grain yield in infertile tropical soil. The overarching objective is to determine how ecosystem services can contribute to the improvement of land-use policy to ensure the sustainability of cultivated lands, in such a way that forest can be preserved by avoiding deforestation of other new areas through shifting cultivation systems. Four leguminous tree species were used, two with high-quality residues Leucaena leucocephala (leucaena) and Gliricidia sepium (gliricidia) and two with low-quality residues Clitoria fairchildiana (clitoria) and Acacia mangium (acacia). Maize grain yield was evaluated between 2011 and 2017 in these treatments. In 2018, to assess how ecosystem services affect crop performance, the treatmentswere divided into ten treatments with and without urea.We conclude that increased uptake of inorganic and organic N by maize resulting from improvement of the soil quality indicators may allow agricultural intensification. This improvement can help meet the challenges of sustainability and feasibility of agroecosystems of the Amazonian periphery by making the agroecosystem more productive year by year. Therefore, our results confirm that the utilization of an ecosystem services style approach can help meet the challenges of sustainability and feasibility in agrosystems of the Amazonian periphery. In addition, these results can contribute to the development of land-use policy in the Amazonian periphery, aiming for the intensification of agriculture in cropped areas to avoid deforestation of new areas from shifting cultivation systems.

Construction of Charring-Functional Polyheptanazine towards Improvements in Flame Retardants of Polyurethane.

Nitrogen-containing flame retardants have been extensively applied due to their low toxicity and smoke-suppression properties; however, their poor charring ability restricts their applications. Herein, a representative nitrogen-containing flame retardant, polyheptanazine, was investigated. Two novel, cost-effective phosphorus-doped polyheptazine (PCN) and cobalt-anchored PCN (Co@PCN) flame retardants were synthesized via a thermal condensation method. The X-ray photoelectron spectroscopy (XPS) results indicated effective doping of P into triazine. Then, flame-retardant particles were introduced into thermoplastic polyurethane (TPU) using a melt-blending approach. The introduction of 3 wt% PCN and Co@PCN could remarkably suppress peak heat release rate (pHRR) (48.5% and 40.0%), peak smoke production rate (pSPR) (25.5% and 21.8%), and increasing residues (10.18 wt%→17.04 wt% and 14.08 wt%). Improvements in charring stability and flame retardancy were ascribed to the formation of P–N bonds and P=N bonds in triazine rings, which promoted the retention of P in the condensed phase, which produced additional high-quality residues.

Microbial metabolic potential to transform plant residual carbon

The pathways of exogenous carbon (C) mineralization and retention in soil have been widely investigated via analyses of microbial fingerprints (e.g., phospholipid fatty acids (PLFAs)) and footprints (e.g., amino sugars). However, the potential of soil resident microbiota to metabolize plant-derived substrates remains unclear. This study aimed to clarify how microbial growth cumulatively contributes to residual C transformation under different substrate qualities and soil management practices. Nonlinear fitting of the cumulative contribution of PLFA-based microbial growth to residual C decomposition showed that among microbial groups, fungi metabolized grains to the greatest extent, and gram-negative bacteria contributed the most to root C metabolism, whereas gram-positive bacteria and actinomycetes were generally less competent in the decomposition of all residues. The sequestration of leaf and root C in amino sugars was greater without tillage than with tillage, whereas an opposite tillage trend was observed for grain-derived galactosamine. Our findings highlight that the potential of fungi and bacteria for residual C transformation depends on residue quality. Furthermore, high-quality residue (e.g., grain) transformation is strongly exacerbated by soil disturbance, whereas few tillage-facilitated effects occur with a decline in substrate quality (e.g., leaf and root).

Cropping systems including legume cover crops favour mineral–organic associations enriched with microbial metabolites in no-till soil

Long-term carbon (C) stabilisation in tropical and subtropical soils under no-tillage (NT) rests on the formation of mineral–organic associations (MOAs) that can be enriched with microbial metabolites. In this work, we assessed the role of long-term tillage and cropping systems and mineral N fertilisation in enriching MOAs with microbial metabolites in a subtropical soil. For this purpose, we sampled a sandy clay loam Acrisol up to 1 m depth involved in an ongoing 30-year-old experiment under two different tillage systems (conventional tillage and NT) in the presence and absence of legume cover crops and mineral nitrogen (N) fertilisation. The soil samples were subjected to particle size fractionation and n-alkane analysis. The NT and the presence of legume cover crops in the surface soil layer (0−5 cm) increased the abundance of plant-derived lipids (i.e. compounds with n-alkane chains of 25−33 C atoms) in the whole soil. Microbial-derived lipids (i.e. compounds with shorter n-alkane chains (15−24 C atoms)) were more abundant in the clay fraction of the surface (0−5 cm) and sub-surface soil layers (20−30 and 75−100 cm) in NT soil receiving high-quality residues of legume cover crops. However, N fertilisation decreased the abundance of microbial-derived lipids in the clay fraction of the 0−5 and 20−30 cm soil layers. Our findings highlight the role of N-rich residues of legume cover crops, but not of mineral N fertilisation, in the long-term stabilisation of C in MOAs in NT soils through the action of microbial residues.

How do earthworms affect decomposition of residues with different quality apart from fragmentation and incorporation?

Earthworms especially contribute to decomposition by fragmenting, incorporating, and mixing residues into the soil. However, residues are already mechanically fragmented and incorporated into the soil by combine harvesters in most arable cropping systems of China, which may diminish the effects of earthworms. How earthworms affect the decomposition of residues with different quality, apart from fragmentation and incorporation, is still not clear. Thus, the aim of the present 60-day soil microcosm incubation experiment was to assess how earthworms affect the decomposition of residues of different quality, aside from fragmentation and incorporation. Four residues with different quality (high quality: rapeseed cake; medium quality: corn leaf; low quality: rice straw and corn stalk) were fragmented to 1-mm pieces using a laboratory blender and then thoroughly mixed with soil. The decomposition rates of different residues with or without earthworms (Metaphire guillelmi) were determined by measuring the CO2 emission. We hypothesized that earthworms accelerate high-quality residue decomposition more than low-quality residues. Results showed that earthworms increased cumulative CO2 emission for each residue type separately. Carbon derived from the residues was calculated and the decomposition kinetics was fitted with the first-order exponential model. During the initial 20 days of the incubation, the decomposition rate constant (k) of low-quality residues was significantly decreased by earthworm addition and significantly increased during the later period (20–60 days) of the incubation. However, no significant difference was detected between earthworm presence and absence treatments on the k value with high- and medium-quality residue incorporation. In general, earthworm addition increased inorganic nitrogen contents in most residue treatments during the entire incubation period, except for the rapeseed cake-treated soil. Our results indicated that earthworms have no effect on the decomposition of high-quality residues, but have a positive effect on the later stages of low-quality residue decomposition.

Tracking the Release of Soil Nitrate and Labile C in A Legume-Maize Rotation in Zimbabwe

This study compared the effect of a weedy fallow (5.2 t/ha biomass), a velvet bean (Mucuna pruriens) cut for hay (7.2 t/ha biomass) and a green-manured M. pruriens (6.49 t/ha biomass) on the dynamics of soil N and C in a maize crop. An on-farm, farmer participatory experiment was established on a farmer’s field in Wedza District, Zimbabwe. Soil mineral N and labile carbon were determined at intervals upto 120 cm depth, at maize planting and at 1 and 2 weeks after planting. Before planting, the soil mineral N content ranged from 28 kg N/ha after weed fallow to 107 kgN/ha following M. pruriens. Total nitrate concentration was highest in the 0-15 cm depth of the M. pruriens treatments in the pre-planting sampling, but following rainfall and maize planting, nitrate concentration declined rapidly. By 2 weeks after planting, 7.5 and 13.5 kg N/ha remained in the 0-120 cm soil depth of the weedy fallow and green-manured M. pruriens, respectively. Improving synchrony of nutrient release and uptake is critical when applying high quality residues which breakdown relatively slowly. This could result in significant inputs of C, release nutrients more slowly and reduce soil nutrient losses.

Food quantity and quality of cassava affected by leguminous residues and inorganic nitrogen application in a soil of low natural fertility of the humid tropics

The aim of this study was to test the hypothesis that the quality and quantity of biofortified cassava root in a humid tropical environment can be modified with the application of a combination of low- and high-quality residues of leguminous tree species. The experiment was designed as a 6 × 2 factorial (a combination of 4 legume species versus 2 levels of nitrogen) with 4 replications in a randomized block design and the following treatments: Gliricidia + Acacia, Gliricidia + Leucaena, Gliricidia + Clitoria, Leucaena + Acacia, Leucaena + Clitoria, and a control without legumes. We analyzed the shoot weight, number of roots/plant, root weight, root production, proximate composition, as well as the mineral, carotenoid, and pro-vitamin A contents. Root production increased with the application of high-quality residues. The protein level influenced the carotenoid content. The allelopathic effect of exotic genera — Leucaena and Acacia —, especially when combined, can decrease the mineral content such as potassium and, therefore, reduce the accumulation of starch.

The RING 2.0 web server for high quality residue interaction networks.

Residue interaction networks (RINs) are an alternative way of representing protein structures where nodes are residues and arcs physico–chemical interactions. RINs have been extensively and successfully used for analysing mutation effects, protein folding, domain–domain communication and catalytic activity. Here we present RING 2.0, a new version of the RING software for the identification of covalent and non-covalent bonds in protein structures, including π–π stacking and π–cation interactions. RING 2.0 is extremely fast and generates both intra and inter-chain interactions including solvent and ligand atoms. The generated networks are very accurate and reliable thanks to a complex empirical re-parameterization of distance thresholds performed on the entire Protein Data Bank. By default, RING output is generated with optimal parameters but the web server provides an exhaustive interface to customize the calculation. The network can be visualized directly in the browser or in Cytoscape. Alternatively, the RING-Viz script for Pymol allows visualizing the interactions at atomic level in the structure. The web server and RING-Viz, together with an extensive help and tutorial, are available from URL: http://protein.bio.unipd.it/ring.

Effects of residue quality and soil mineral N on microbial activities and soil aggregation in a tropical sandy soil in Senegal

The role played by organic residues and exogenous mineral N in the formation of stable soil aggregates in nutrient-poor, tropical sandy soils of Senegal is relatively unclear. This study assessed the effect of two representative low and high quality residues (Zea mays and Crotalaria retusa respectively) on the formation and stability of soil aggregates. The formation and stability of aggregates, soil biomass, root biomass, fungal hyphae length, C mineralization and chitinase activity, as a specific biomarker of the activity of fungal populations, were measured under controlled conditions over 120 days. In both the control and amended soils, there were more macroaggregates (>2000 μm) and mesoaggregates (250–2000 μm) than microaggregates (50–250 μm and <50 μm). The formation of macroaggregates and stability (MWD) were not significantly affected by the quality of residues. Amendment with organic residues shifted the distribution of the aggregate fractions. The macroaggregates increased by 26% with crotalaria and by 35% with maize residues while mesoaggregates decreased by 18% with crotalaria and by 26% with maize residues and microaggregates decreased by 8% with crotalaria and by 9% with maize residues. This study also confirmed that macroaggregates are formed from micro- and mesoaggregates. The total microbial biomass was significantly higher in soil amended with maize residues compared to soil with crotalaria residues and the control soil although the fungal hyphae length decreased when the soil was amended with either crop residue. Chitinase activity is the most pertinent indicator associated with macroaggregation stability. Adding mineral N (equivalent to 120 kg N ha−1 as urea) to the residue increased microbial biomass and reduced fungal hyphae length but had no effect on macroaggregate formation and fungal activity. These observations suggested that, for short term incubation of soil amended with residues, fungal activity plays a greater role in aggregation in sandy soils than the fungal population density.

High quality residues from cover crops favor changes in microbial community and enhance C and N sequestration

The objective of the study was to evaluate the effect of a change in management on the soil microbial community and C sequestration. We conducted a 3-year field study in La Pampa (Argentina) with rotation of sorghum (Sorghum bicolor) in zero tillage alternating with rye (Secale cereale) and vetch (Vicia villosa ssp. dasycarpa). Soil was sampled once a year at two depths. Soil organic matter fractions, dissolved organic matter, microbial biomass (MBC) and community composition (DNA extraction, qPCR, and phospholipid FAME profiles) were determined. Litter, aerial- and root biomass were collected and all material was analyzed for C and N. Results showed a rapid response of microbial biomass to a bacterial dominance independent of residue quality. Vetch had the highest diversity index, while the fertilized treatment had the lowest one. Vetch–sorghum rotation with high N mineralization rates and diverse microbial community sequestered more C and N in stable soil organic matter fractions than no-till sorghum alone or with rye, which had lower N turnover rates. These results reaffirm the importance of enhanced soil biodiversity for maintaining soil ecosystem functioning and services. The supply of high amounts of N-rich residues as provided by grass–legume cover crops could fulfill this objective.

Amendment ofTephrosiaImproved Fallows with Inorganic Fertilizers Improves Soil Chemical Properties, N Uptake, and Maize Yield in Malawi

Maize production in Malawi is limited mainly by low soil N and P. Improved fallows of N-fixing legumes such asTephrosiaandSesbaniaoffer options for improving soil fertility particularly N supply. The interactions ofTephrosiafallows and inorganic fertilizers on soil properties, N uptake, and maize yields were evaluated at Chitedze Research Station in Malawi. The results indicated that the level of organic matter and pH increased in all the treatments except for the control. Total N remained almost unchanged while available P decreased in all plots amended withT. vogeliibut increased inT. candidaplots where inorganic P was applied. Exchangeable K increased in all the plots irrespective of the type of amendment. The interaction of N and P fertilizers withT. vogeliifallows significantly increased the grain yield. The treatment that received 45 kg N ha−1and 20 kg P ha−1produced significantly higher grain yields (6.8 t ha−1) than all the other treatments except where 68 kg N ha−1and 30 kg P ha−1were applied which gave 6.5 t ha−1of maize grain.T. candidafallows alone or in combination with N and P fertilizers did not significantly affect grain yield. However,T. candidafallows alone can raise maize grain yield by 300% over the no-input control. Based on these results we conclude that high quality residues such asT. candidaandT. vogeliican be used as sources of nutrients to improve crop yields and soil fertility in N-limited soils. However, inorganic P fertilizer is needed due to the low soil available P levels.

Maize Residue Interaction with High Quality Organic Materials: Effects on Decomposition and Nutrient Release Dynamics

The application of organic materials with wide carbon-to-nitrogen (C-to-N) ratios is known to cause initial immobilization of nutrients, unless N fertilizers are applied. In Sub-Saharan Africa, regular application of mineral fertilizers with organic residues is seldom practiced due to several socioeconomic constraints. In the present study, we assessed the biomass qualities of Zea mays, Tithonia diversifolia and Vicia faba and evaluated whether delayed decomposition and nutrient release of low quality residues will improve when mixed with high quality residues. Our hypothesis was that high quality organic residues have high N supply capabilities to improve decomposition and nutrient release of low quality materials when mixed together. Compared with V. faba and T. diversifolia biomasses, Z. mays residue was found to be relatively poor in quality as a result of its relatively low N concentration (10.8 g/kg) and wider C-to-N ratio (37.2:1). The assessment on biomass quality was consistent with the results on decomposition. After one week, 42 % of Z. mays residues had decomposed compared with more than 90 % of T. diversifolia and V. faba residues. Further, the decomposition and N release rate of Z. mays tripled when mixed with either T. diversifolia or V. faba biomass. In this study, the initial N, C, lignin, C-to-N ratio, lignin-to-N ratio and (lignin + polyphenol)/N ratio of the residues were useful indicators of degradability and nutrient release based on significant (P < 0.005) correlations. The study found that mixing Z. mays residue with either T. diversifolia or V. faba green biomass improved the N composition and C-to-N ratio of the mixture, which accounted for the improved decomposition and nutrient release rates of Z. mays residues in the mixture compared with sole Z. mays treatment. Our study therefore suggested that in places where inorganic fertilizers are limited, T. diversifolia and V. faba residues could be viable sources of N for improved decomposition and nutrient release of low quality residues.