Microbial biomass and activity in litter during the initial development of pure and mixed plantations of Eucalyptus grandis and Acacia mangium Biomassa e atividade microbiana da serapilheira durante o desenvolvimento inicial de plantios puros e mistos de Eucalyptus grandis e Acacia mangium

Studies on microbial activity and biomass in forestry plantations often overlook the role of litter, typically focusing instead on soil nutrient contents to explain plant and microorganism development. However, since the litter is a significant source of recycled nutrients that affect nutrient dynamics in the soil, litter composition may be more strongly correlated with forest growth and development than soil nutrient contents. This study aimed to test this hypothesis by examining correlations between soil C, N, and P; litter C, N, P, lignin content, and polyphenol content; and microbial biomass and activity in pure and mixed second-rotation plantations of Eucalyptus grandis and Acacia mangium before and after senescent leaf drop. The numbers of cultivable fungi and bacteria were also estimated. All properties were correlated with litter C, N, P, lignin and polyphenols, and with soil C and N. We found higher microbial activity (CO2 evolution) in litter than in soil. In the E. grandis monoculture before senescent leaf drop, microbial biomass C was 46 % higher in litter than in soil. After leaf drop, this difference decreased to 16 %. In A. mangium plantations, however, microbial biomass C was lower in litter than in soil both before and after leaf drop. Microbial biomass N of litter was approximately 94 % greater than that of the soil in summer and winter in all plantations. The number of cultivable fungi and bacteria increased after leaf drop, especially so in the litter. Fungi were also more abundant in the E. grandis litter. In general, the A. mangium monoculture was associated with higher levels of litter lignin and N, especially after leaf drop. In contrast, the polyphenol and C levels in E. grandis monoculture litter were higher after leaf drop. These properties were negatively correlated with total soil C and N. Litter in the mixed stands had lower C:N and C:P ratios and higher N, P, and C levels in the microbial biomass. This suggests more effective nutrient cycling in mixed plantations in the long term, greater stimulation of microbial activity in litter and soil, and a more sustainable system in general.A serapilheira é, muitas vezes, um compartimento negligenciado para avaliação e melhor entendimento do comportamento da atividade e biomassa microbiana em plantios florestais. Quase sempre, em estudos dessa natureza, os autores tentam encontrar explicações para o padrão de desenvolvimento de plantas e microrganismos, por meio da avaliação dos nutrientes minerais no solo. Entretanto, considerando-se a dinâmica de disponibilização desses nutrientes, a hipótese é que provavelmente ocorra maior relação entre o crescimento de plantas florestais e da atividade microbiana com os nutrientes presentes na serapilheira. Este estudo teve como objetivo avaliar a atividade e biomassa microbiana e o número mais provável de fungos e bactérias totais cultiváveis do solo e da serapilheira, numa segunda rotação de plantios puros e mistos de Eucalyptus grandis e Acacia mangium, manejados em cultivo mínimo, antes e após a deposição de folhas senescentes. Esses atributos foram relacionados com os teores totais de C, N, P e teor de lignina e polifenóis da serapilheira e C, N e P do solo. Antes da deposição de folhas senescentes, o C da biomassa microbiana da serapilheira foi 46 % maior que a biomassa microbiana do solo nos plantios de E. grandis. Após a deposição foliar, essa diferença diminuiu para 16 %. No entanto, o C da biomassa microbiana da serapilheira no plantio puro de A. mangium foi menor que qualquer um desses valores. Já o N da biomassa microbiana da serapilheira apresentou-se com grande potencial para estocar esse nutriente, sendo, em média, 94 % maior que o N da biomassa microbiana do solo nas duas épocas e em todos os plantios. Houve maior atividade microbiana na serapilheira que no solo. Além disso, o plantio misto e a monocultura de E. grandis com N apresentaram maior atividade microbiana, após a deposição foliar. O número mais provável de fungos e bactérias totais cultiváveis foi maior após a deposição, com maior representação na serapilheira; os fungos foram mais relacionados com os resíduos de E. grandis. De maneira geral, o plantio puro de A. mangium foi mais relacionado com altos teores de lignina e N na serapilheira, principalmente após a deposição foliar. Diferentemente, as monoculturas de E. grandis sempre tiveram relação com os maiores teores de C da serapilheira e de fenóis totais, após a deposição foliar, sendo negativamente relacionadas com os teores de C e N totais do solo. O plantio consorciado promoveu na serapilheira menores relações C:N e C:P e maiores teores de N, P e C da biomassa microbiana, sobretudo após a deposição foliar, o que demonstra a dinâmica desse compartimento. Esse fato pode favorecer melhor ciclagem de nutrientes em longo prazo, com estímulo à atividade microbiana do solo e da serapilheira e à sustentabilidade desse sistema de plantio

Microbial biomass and activity in litter during the initial development of pure and mixed plantations of Eucalyptus grandis and Acacia mangium

Studies on microbial activity and biomass in forestry plantations often overlook the role of litter, typically focusing instead on soil nutrient contents to explain plant and microorganism development. However, since the litter is a significant source of recycled nutrients that affect nutrient dynamics in the soil, litter composition may be more strongly correlated with forest growth and development than soil nutrient contents. This study aimed to test this hypothesis by examining correlations between soil C, N, and P; litter C, N, P, lignin content, and polyphenol content; and microbial biomass and activity in pure and mixed second-rotation plantations of Eucalyptus grandis and Acacia mangium before and after senescent leaf drop. The numbers of cultivable fungi and bacteria were also estimated. All properties were correlated with litter C, N, P, lignin and polyphenols, and with soil C and N. We found higher microbial activity (CO2 evolution) in litter than in soil. In the E. grandis monoculture before senescent leaf drop, microbial biomass C was 46 % higher in litter than in soil. After leaf drop, this difference decreased to 16 %. In A. mangium plantations, however, microbial biomass C was lower in litter than in soil both before and after leaf drop. Microbial biomass N of litter was approximately 94 % greater than that of the soil in summer and winter in all plantations. The number of cultivable fungi and bacteria increased after leaf drop, especially so in the litter. Fungi were also more abundant in the E. grandis litter. In general, the A. mangium monoculture was associated with higher levels of litter lignin and N, especially after leaf drop. In contrast, the polyphenol and C levels in E. grandis monoculture litter were higher after leaf drop. These properties were negatively correlated with total soil C and N. Litter in the mixed stands had lower C:N and C:P ratios and higher N, P, and C levels in the microbial biomass. This suggests more effective nutrient cycling in mixed plantations in the long term, greater stimulation of microbial activity in litter and soil, and a more sustainable system in general