Soil microstructure alterations induced by land use change for sugarcane expansion in Brazil

© 2019 British Society of Soil Science Land use change (LUC) alters soil structure and, consequently, the functions and services provided by these soils. Conversion from extensive pasture to sugarcane is one of the largest land transitions in Brazil as a result of the growth of the domestic and global demands of bioenergy. However, the impacts of sugarcane expansion on the soil structure under extensive pasture remains unclear, especially when considering changes at the microscale. We investigated whether LUC for sugarcane cultivation impacted soil microstructure quality. Undisturbed soil samples were taken from two soil layers (0–10 and 10–20cm) under three contrasting land uses (native vegetation—NV, pasture—PA and sugarcane—SC) in three different locations in the central-southern Brazil. Oriented thin sections (30μm) were used for micromorphological analysis. The total area of pores decreased following the LUC in the following order: NV > PA > SC in both soil layers. The area of large complex packing pores (>0.01mm²) also decreased with the LUC sequence: NV>PA>SC. Qualitative and semi-quantitative micromorphological analysis confirmed porosity reduction was driven by the decrease in complex packing pores and that biological features decreased in the same LUC sequence as the quantitative parameters. Therefore, LUC for sugarcane expansion reduced microscale soil porosity, irrespectively of soil type and site-specific conditions, indicating that the adoption of more sustainable management practices is imperative to preserve soil structure and sustain soil functions in Brazilian sugarcane fields

High application rates of biochar to mitigate N2O emissions from a N-fertilized tropical soil under warming conditions.

Biochar application has been suggested as a strategy to decrease nitrous oxide emissions from agricultural soils while increasing soil C stocks, especially in tropical regions. Climate change, specifically increasing temperatures, will affect soil environmental conditions and thereby directly influence soil N2O fluxes

Maize-Brachiaria intercropping: A strategy to supply recycled N to maize and reduce soil N₂O emissions?

Nitrogen use in agriculture directly impacts food security, global warming, and environmental degradation. Forage grasses intercropped with maize produce feed for animals and or mulch for no-till systems. Forage grasses may exude nitrification inhibitors. It was hypothesized that brachiaria intercropping increases N recycling and maize grain yield and reduces nitrous oxide (N2O) emissions from soil under maize cropping. A field experiment was set up in December 2016 to test three cropping system (maize monocropped, maize intercropped with Brachiaria brizantha or with B. humidicola) and two N rates (0 or 150 kg ha-1). The grasses were sown with maize, but B. humidicola did not germinate well in the first year. B. brizantha developed slowly during the maize cycle because of shading but expanded after maize was harvested. The experiment was repeated in 2017/2018 when B. humidicola was replanted. N2O and carbon dioxide (CO2) emissions, maize grain yield and N content were measured during the two seasons. After the first maize harvest, the above- and below-ground biomass, C and N content of B. brizantha grown during fall-winter, and the biological nitrification inhibition potential of B. brizantha were evaluated. Maize yield responded to N fertilization (5.1 vs. 9.8 t ha-1) but not to brachiaria intercropping. B. brizantha recycled approximately 140 kg N ha-1 and left 12 t dry matter ha-1 for the second maize crop. However, the 2017/18 maize yields were not affected by the N recycled by B. brizantha, whereas N2O emissions were higher in the plots with brachiaria, suggesting that part of the recycled N was released too early after desiccation. Brachiarias showed no evidence of causing nitrification inhibition. The strategy of intercropping brachiarias did not increase maize yield, although it added C and recycled N in the system

Legacy effects of intercropping and nitrogen fertilization on soil N cycling, nitrous oxide emissions, and the soil microbial community in tropical maize production

Maize-forage grasses intercropping systems have been increasingly adopted by farmers because of their capacity to recycle nutrients, provide mulch, and add C to soil. However, grasses have been shown to increase nitrous oxide (N2O) emissions. Some tropical grasses cause biological nitrification inhibition (BNI) which could mitigate N2O emissions in the maize cycle but the reactions of the N cycle and the microbial changes that explain the N2O emissions are little known in such intercropping systems. With this in mind, we explored intercropping of forage grasses (Brachiaria brizantha and Brachiaria humidicola) with distinct BNI and yield potential to increase N cycling in no-till maize production systems compared to monocrop with two N rates (0 and 150 kg ha−1) applied during the maize season. These grasses did not strongly compete with maize during the period of maize cycle and did not have a negative effect on grain yield. We observed a legacy of these grasses on N mineralization and nitrification through the soil microbiome during maize growth. We observed that B. humidicola, genotype with higher BNI potential, increased net N mineralization by 0.4 mg N kg−1 day−1 and potential nitrification rates by 1.86 mg NO3-N kg−1 day−1, while B. brizantha increased the soil moisture, fungi diversity, mycorrhizal fungi, and bacterial nitrifiers, and reduced saprotrophs prior to maize growth. Their legacy on soil moisture and cumulative organic inputs (i.e., grass biomass) was strongly associated with enhanced mineralization and nitrification rates at early maize season. These effects contributed to increase cumulative N2O emission by 12.8 and 4.8 mg N2O-N m−2 for maize growing after B. brizantha and B. humidicola, respectively, regardless of the N fertilization rate. Thus, the nitrification inhibition potential of tropical grasses can be outweighed by their impacts on soil moisture, N recycling, and the soil microbiome that together dictate soil N2O fluxes

Legacy effects of intercropping and nitrogen fertilization on soil N cycling, nitrous oxide emissions, and the soil microbial community in tropical maize production

Maize-forage grasses intercropping systems have been increasingly adopted by farmers because of their capacity to recycle nutrients, provide mulch, and add C to soil. However, grasses have been shown to increase nitrous oxide (N2O) emissions. Some tropical grasses cause biological nitrification inhibition (BNI) which could mitigate N2O emissions in the maize cycle but the reactions of the N cycle and the microbial changes that explain the N2O emissions are little known in such intercropping systems. With this in mind, we explored intercropping of forage grasses (Brachiaria brizantha and Brachiaria humidicola) with distinct BNI and yield potential to increase N cycling in no-till maize production systems compared to monocrop with two N rates (0 and 150 kg ha−1) applied during the maize season. These grasses did not strongly compete with maize during the period of maize cycle and did not have a negative effect on grain yield. We observed a legacy of these grasses on N mineralization and nitrification through the soil microbiome during maize growth. We observed that B. humidicola, genotype with higher BNI potential, increased net N mineralization by 0.4 mg N kg−1 day−1 and potential nitrification rates by 1.86 mg NO3-N kg−1 day−1, while B. brizantha increased the soil moisture, fungi diversity, mycorrhizal fungi, and bacterial nitrifiers, and reduced saprotrophs prior to maize growth. Their legacy on soil moisture and cumulative organic inputs (i.e., grass biomass) was strongly associated with enhanced mineralization and nitrification rates at early maize season. These effects contributed to increase cumulative N2O emission by 12.8 and 4.8 mg N2O-N m−2 for maize growing after B. brizantha and B. humidicola, respectively, regardless of the N fertilization rate. Thus, the nitrification inhibition potential of tropical grasses can be outweighed by their impacts on soil moisture, N recycling, and the soil microbiome that together dictate soil N2O fluxes

Challenges and approaches in assessing the interplay between microorganisms and their physical micro-environments

© 2020 The Author(s) Spatial structure over scales ranging from nanometres to centimetres (and beyond) varies markedly in diverse habitats and the industry-relevant settings that support microbial activity. Developing an understanding of the interplay between a structured environment and the associated microbial processes and ecology is fundamental, but challenging. Several novel approaches have recently been developed and implemented to help address key questions for the field: from the use of imaging tools such as X-ray Computed Tomography to explore microbial growth in soils, to the fabrication of scratched materials to examine microbial-surface interactions, to the design of microfluidic devices to track microbial biofilm formation and the metabolic processes therein. This review discusses new approaches and challenges for incorporating structured elements into the study of microbial processes across different scales. We highlight how such methods can be pivotal for furthering our understanding of microbial interactions with their environments

Greenhouse gases emission after in natura and anaerobic digested vinasse application in soil

O processamento anaeróbio da vinhaça pode ser útil para reduzir as emissões de gases do efeito estufa após a aplicação do seu efluente, com menos C lábil, em solos. O presente trabalho mediu as emissões de gases de efeito estufa através do método de câmaras estáticas após a aplicação das duas formas de vinhaça (in natura e processada anaerobiamente ) e a interação com ureia em um LATOSSOLO VERMELHO simulando a camada 0-20 cm de um solo em uma coluna de PVC. Foi observado que a aplicação de vinhaça processada ao solo emitiu 2.592 µg C-CH4 m-2 ao final de 90 dias, mais do que os tratamentos com vinhaça processada com ureia (-590 µg C-CH4 m-2), controle (-601 g C-CH4 m-2), vinhaça in natura (-1.176 µg C-CH4m-2), vinhaça in natura com ureia (-1.533 µg C-CH4 m-2), e ureia (-2.760 µg C-CH4 m-2). A adição de vinhaça in natura estimulou a emissão de CO2 produzindo 60,2 g C-CO2 m-2 ao longo de 90 dias, valor superior aos tratamentos vinhaça processada com ureia (36,7 g C-CO2 m-2), ureia (35,1 g C-CO2m-2), controle (25,8 g C-CO2m-2) e vinhaça processada (22,1 g C-CO2 m-2). A adição de vinhaça in natura com ureia (45,0 g C-CO2 m-2) emitiu mais CO2 em relação aos tratamentos controle e vinhaça processada, além disso, a aplicação da ureia junto da vinhaça processada provocou um aumento na emissão de CO2 em relação a vinhaça processada sozinha. Os maiores fluxos de N2O foram observados no solo fertilizado com vinhaça processada com ureia (302,8 mg N-N2O m-2) e apenas ureia (199,4 mg N-N2 m-2), seguido da fertilização com vinhaça in natura com ureia (70,9 mg N- N2O m-2) superior aos tratamentos com vinhaça processada (12,14mg N-N2O m-2), controle (7,54 mg N-N2O m-2) e vinhaça in natura (6,64 mg N- N2O m-2). Com a realização das primeiras medições das emissões de gases do efeito estufa após a aplicação da vinhaça processada no solo concluiu-se que as emissões de CO2 são mitigadas, porém provoca aumento fluxo de CH4 e N2O do solo para a atmosfera em relação à vinhaça in natura.The vinasse anaerobic digestion migh be usefull to avoid greenhouse gases emissions after apply the effluent on soil due the low labile C content. This research measured during 90 days the greenhouse gases emissions using the static chamber method after the application of two forms of vinasse (in natura and digested) and the interaction of both with urea in a soil PVC column simulating the 0-20 cm layer of a Red Latossol. The digested vinasse application in the soil produced 2,592 µg C-CH4 m-2 at the end of 90 days, more than soil with digested vinasse with urea (-590 µg C-CH4 m-2), control (-601 µg C-CH4 m-2), vinasse in natura (-1,176 µgC-CH4m-2), vinasse in natura with urea (-1,533 µg C-CH4 m-2), urea (-2,760 µg C-CH4 m-2). Vinasse in natura induced soil CO2 emission, producing 60.2 g C-CO2 m-2 at the end of 90 days, greater than digested vinasse with urea (36.7 g C-CO2 m-2), urea (35.1 g C-CO2 m-2), control (25.8 g C-CO2 m-2), and diested vinasse (22.1 g C-CO2 m-2). The interaction between vinasse in natura and urea in the soil emitted (45.0 g C-CO2 m-2) more CO2 than soil control and digested vinasse, furthermore the interaction between urea and digested vinasse resulted in a greater CO2 release to atmosphere than only digested vinasse. The greater N2O efflux was provided by digested vinasse with urea (302.8 mg N- N2O m-2) and only urea (199.4 mg N-N2O m-2) application, followed by vinasse in natura (70.9), which emitted more nitrous oxide than digested (12.14 mg N-N2O m-2), control (7.54 mg N- N2O m-2), and vinasse in natura (6.64 mg N- N2O m-2). After this first measurement of greenhouse gases efflux after soil fertilization with anaerobic digested vinasse was observed a CO2 mitigation, however the CH4 and N2O efflux from soil to atmosphere can be greater than vinasse in natura

Deforestation control and the impact on agricultural income in Brazil

O crescimento da produção agropecuária contribui para discussões relacionadas ao desmatamento das florestas brasileiras, que tem registrado elevadas taxas nos últimos anos. Frequentemente, o crescimento do desflorestamento é atribuído aos retornos do setor agropecuário, com o aumento da produção pautado pela expansão de terras. Neste contexto, o principal objetivo desta pesquisa é investigar se a implantação de medidas de redução do desmatamento também reduz a renda do setor agropecuário brasileiro. Para isso, foram simulados três cenários de política de redução do desmatamento através do uso de um modelo inter-regional (bottom-up) e dinâmico-recursivo de equilíbrio geral computável, o TERM-BR, que utiliza uma matriz de transição para capturar as mudanças no uso da terra entre as categorias da agropecuária (culturas, pastagens e silvicultura) e um tipo de uso residual representando as florestas nativas (unused). Os cenários de desmatamento utilizados aqui são originais do estudo do Instituto Escolhas (2017), com o cenário base projetando as taxas de desmatamento seguindo a tendência recente da série histórica, e três cenários de política: ZERO, que projeta a interrupção total do desflorestamento nas florestas brasileiras até 2030; CEN2, impõem o fim do desmatamento ilegal em 2030, mas com o desmatamento em terras privadas prosseguindo em áreas de elevada aptidão agrícola; e CEN3, que é semelhante ao cenário CEN2, mas o desflorestamento em terras privadas ocorre sobre qualquer tipo de terra, independente da aptidão agrícola. Os resultados indicaram que a redução do desmatamento no Brasil não reduz a renda agropecuária, inclusive, as simulações apontaram para um ganho moderado na renda agregada do setor. No cenário mais restrito (ZERO), os resultados apontaram para elevação de 1,13% na renda agropecuária, com crescimento de 0,61% e 0,12% nos cenários CEN2 e CEN3, respectivamente. Este crescimento ocorre devido ao aumento nos rendimentos da terra, que se torna escassa com a aplicação da política e com a elevação dos preços dos alimentos e matérias-primas, que elevam os valores da produção. Apesar disso, os resultados regionais mostraram que os estados pertencentes à região da Amazônia Legal registraram queda na renda agropecuária, enquanto os estados mais afastados dessa região tiveram suas rendas elevadas. Outro resultado importante refere-se ao setor da agroindústria que, diferente do setor agropecuário, registrou queda na renda agregada com a implantação das políticas. Ademais, foi observado que a implantação das políticas causou um custo social ao reduzir o consumo das famílias mais pobres da economia, devido à elevação dos preços dos alimentos e queda dos salários reais. O estudo também constatou que uma elevação de produtividade relativamente pequena é necessária para manter os níveis de produção anterior à aplicação das políticas, com crescimento de 1,4% até 2030 no cenário ZERO. De forma geral, apesar dos resultados do estudo apontarem que a redução do desmatamento não prejudica os rendimentos do setor agropecuário, os resultados negativos em termos regionais e sociais se mostraram importantes para condução deste tipo de política, que podem guiar medidas compensatórias para que os impactos para estes agentes possam ser minimizados.The growth of agricultural production contributes to discussions related to deforestation of Brazilian forests, which has registered high rates recently. Often, the growth of deforestation is attributed to the returns of the agricultural sector, with the increase in production guided by the expansion of land. In this context, the main objective of this research is to investigate whether the implementation of measures to reduce deforestation also reduces the income of the Brazilian agricultural sector. For this, three scenarios of deforestation reduction policy were simulated using an interregional, bottom-up, dynamic general equilibrium model, the TERM-BR model, which uses a transition matrix to capture changes in land use between the categories of agriculture (crops, pastures and forestry) and a type of residual use representing native forests (unused). The deforestation scenarios used here are original from Instituto Escolhas (2017), with the base scenario projecting deforestation rates following the recent trend of the historical series, and more three policy scenarios: ZERO, which projects the total interruption of deforestation in Brazilian forests by 2030; CEN2, impose the end of illegal deforestation in 2030, but with deforestation on private land continuing in areas of high agricultural aptitude; and CEN3, which is similar to the CEN2 scenario, but deforestation on private land occurs on any type of land, regardless of agricultural aptitude. The results indicated that the reduction of deforestation in Brazil does not reduce agricultural income. Actually, the simulations pointed to a moderate gain in the aggregate income of the sector. In the more restricted scenario (ZERO), the results pointed to an increase of 1.13% in agricultural income, with growth of 0.61% and 0.12% in the scenarios CEN2 and CEN3, respectively. This growth occurs due to the increase in returns to land, which becomes scarce with the application of the policy and with the increase in prices of food and raw materials, which raise the values of production. Nevertheless, the regional results showed that the states belonging to the region of the Legal Amazon recorded a fall in agricultural income, while the states farther away from this region had increased incomes. Another important result refers to the agroindustry sector, which, unlike the agricultural sector, recorded a fall in aggregate income with the implementation of policies. Moreover, it was observed that the implementation of the policies caused a social cost by reducing the consumption of the poorest families of the economy, due to the rise in food prices and falling real wages. The study also found that a relatively small productivity increase would be necessary to maintain production levels prior to the implementation of the policy, with growth of 1.4% by 2030 in the ZERO scenario. In general, despite the results of the study point out that the reduction of deforestation does not harm the incomes of the agricultural sector, the negative results in regional and social terms proved important for conducting this type of policy, which can guide compensatory measures so that the impacts for these agents are minimized

Soil carbon and nutrients stocks in areas under intensified pasture and expansion of sugar cane in São Paulo State

A capacidade de adicionar carbono orgânico no solo, chamada de sequestro de carbono, vem sendo estudada como uma das formas de mitigação das mudanças climáticas. Esse processo aumenta os estoques do elemento no solo e pode ser decorrente de mudanças no uso e manejo da terra. O acúmulo não só de carbono, mas de outros nutrientes podem alterar atributos físicos e químicos do solo e podem ser utilizados como indicadores de qualidade do solo. Neste estudo, foi avaliado como o manejo e a mudança nos usos da terra afetaram os estoques de carbono e nutrientes do solo, os resultados foram comparados com suas respectivas áreas de vegetação nativa. Assim, esse trabalho fornece subsídios consistentes para auxiliar na avaliação da sustentabilidade dos sistemas de produção agrícola. Neste sentido, objetivo deste estudo foi investigar como os diferentes usos e manejos do solo influenciam a dinâmica dos nutrientes no solo, através de uma avaliação detalhada nos estoques de carbono, nitrogênio, cálcio, magnésio e potássio e no teor de fósforo orgânico. As etapas do trabalho compreenderam estimar os estoques de carbono e macronutrientes mencionados acima em solos sob vegetação nativa (VN) (floresta Ombrófila Semidecídua Estacional), cana-de-açúcar (CN), pastagem com baixa lotação animal (PE) e pastagem intensiva com alta lotação animal (PI) em duas áreas experimentais localizadas respectivamente na Unidade de Pesquisa de Desenvolvimento de Brotas, da Agência Paulista de Tecnologia dos Agronegócios (APTA, Brotas, SP, Brasil) e a segunda, localizada em Salto de Pirapora, próxima ao campus da Universidade Federal de São Carlos (UFSCAR, Sorocaba, SP, Brasil). Amostras de solo foram coletadas até 50 cm de profundidade. Em Brotas, os estoques de todos os elementos foram menores na floresta, com a mudança no uso da terra houve ganho de aproximadamente 19,7; 19,5 e 14 Mg ha-1 de C, 1,6; 1,5 e 1,2 Mg ha-1 de N, 1,4; 1,6 e 1,3 Mg ha-1 de Ca, 0,8; 1,0 e 0,7 Mg ha-1 de Mg e 0,3; 0,2 e 0,25 Mg ha-1 de K, respectivamente nos tratamentos PE, PI e CN, em relação a vegetação nativa da área na camada de 0-50 cm. Em Salto de Pirapora, observamos o oposto, com a mudança no uso da terra a maioria dos estoques foram maiores na floresta, o saldo foi de aproximadamente 2,1; -2,4 e 4,5 Mg ha-1 de C, 1,9; 1,3 e 1,8 Mg ha-1 de N, 3,4; 3,1 e 2,5 Mg ha-1 de Ca, 0,4; 0,16 e 0,2 Mg ha-1 de Mg e 0,3; 0,08 e 0,25 Mg ha-1 de K, respectivamente nos tratamentos PE, PI e CN, em relação a vegetação nativa da área na camada de 0-50 cm. A área de vegetação nativa localizada em Brotas sofreu perturbação, além disso, os solos das duas áreas experimentais são distintos, impossibilitando a comparação entre elas e dificultando a comparação dos resultados obtidosThe ability to store organic carbon into the soil, called carbon sequestration, has been studied as one of the ways to mitigate climate change. This process increases the element\'s stocks in the soil and may be affected by changes in land use and management. The accrual of not only carbon but other elements in the soil can alter the physical and chemical attributes of the soil and can be used as an indicator of soil quality. This study assessed how the management and change in land uses affect the carbon and nutrient stocks of the soil; the results were compared with their respective areas of native vegetation. Thus, this study provides consistent background to assist in assessing the sustainability of agricultural production systems. Therefore, the objective of this study was to investigate how the different land uses and managements influence the soil nutrients dynamics, through a detailed evaluation of carbon, nitrogen, calcium, magnesium, and potassium stocks and organic phosphorus content. This work aimed to estimate the carbon and macronutrient stocks in soils under native vegetation (VN) (Ombrophylous Semideciduous Seasonal Forest), sugarcane (CN), low-input pastures (PE), and intensive pasture with high animal stocking (PI) in two experimental areas located respectively in the Research and Development Unit of Brotas, of the Agência Paulista de Tecnologia dos Agronegócios (APTA, Brotas, SP, Brazil); the second, located in Salto de Pirapora, near the campus of the Universidade Federal de São Carlos (UFSCAR, Sorocaba, SP, Brazil). Soil samples were collected up to 50 cm deep. In Brotas, all of the elements stocks were lower in the forest, with the change in land use there was an increase of approximately 19,7; 19,5 and 14 Mg ha-1 of C, 1,6; 1,5 and 1,2 Mg ha-1 of N, 1,4; 1,6 and 1,3 Mg ha-1 of Ca, 0,8; 1,0 and 0,7 Mg ha-1 of Mg and 0,3; 0,2 and 0,25 Mg ha-1 of K, respectively in the PE, PI, and CN treatments, compared to the native vegetation of the area in the 0-50 cm. In Salto de Pirapora, was observed the opposite pattern, with the change in land use most of the stocks were higher in the forest and the balance was approximately 2,1; -2,4 and 4,5 Mg ha-1 of C, 1,9; 1,3 and 1,8 Mg ha-1 of N, 3,4; 3,1 and 2,5 Mg ha-1 of Ca, 0,4; 0,16 and 0,2 Mg ha-1 of Mg and 0,3; 0,08 and 0,25 Mg ha-1 of K, respectively in the PE, PI, and CN treatments, compared to the native vegetation of the area in the 0-50 cm. The area of native vegetation located in Brotas has suffered disturbance, in addition, the soils of the two experimental areas are different, making it impossible to compare them and making it difficult to compare the results obtaine

Alterations in soil pH emerge as a key driver of the impact of global change on soil microbial nitrogen cycling: Evidence from a global meta‐analysis

Aim: Soil nitrogen (N) cycling is critical to the productivity of terrestrial ecosystems. However, the impact of global change factors (GCFs) on the microbial mediators of N cycling pathways has yet to be synthesized, and it also remains unclear whether the response of the abundance of N-cycling genes can predict changes in their corresponding processes. Location: Global. Time period: 2000–2021. Major taxa studied: Archaea, bacteria. Methods: We synthesized 8322 paired observations of soil microorganisms related to N cycling from field experiments in which GCFs (climate change and nutrient addition) were manipulated. Results: We found that the abundance of soil microbes and most N-cycling genes were resistant to elevated CO2, experimental warming and water addition/reduction; however, N addition and the combination of N addition with other GCFs significantly increased the abundance of ammonia oxidizer bacteria (amoA-AOB). The results indicated that in steady-state (natural) conditions, the main factors driving the global abundance of soil bacteria, archaea and N-cycling genes varied in terms of the contributions of climatic and edaphic factors. However, upon manipulation of GCFs, the induced change in soil pH was the most essential factor associated with changes in the abundance of soil microbes and N-cycling genes. Notably, the changes in ammonia-oxidizing archaea (amoA-AOA) and amoA-AOB genes, in addition to genes involved in denitrification (nirS and nirK), were significantly correlated with the rates of their corresponding processes, but GCF-induced shifts in the potential nitrification rate (PNR) were explained well by changes in the abundance of the amoA-AOB gene under GCFs. Main conclusions: Our study highlights how ongoing GCFs impact the abundance of soil microbes and N-cycling genes, which might have a profound impact on terrestrial N cycling. Our field-based results provide new insights into the drivers of the abundance of soil microbes and N-cycling genes