Efeito da aplicação de fertilizantes e de cinza de biomassa florestal na instalação de plantações de pinheiro bravo

Abstract

Pretende-se avaliar os efeitos nas plantas e no solo, da cinza de biomassa florestal, em alternativa aos fertilizantes industriais clássicos (N e P), quando aplicados na instalação de um povoamento de Pinus pinaster. Assim, instalou-se em Março de 2005 uma área experimental situada em Pedrógão Grande, com um delineamento experimental de blocos casualizados, composto por três blocos e cinco tratamentos, que consistiram na aplicação de cinza (CI), cinza mais azoto (CIN), fósforo (FP), fósforo mais azoto (FPN) e controlo, sem qualquer aplicação (CT). Efectuaram-se amostragens dos solos antes da plantação e após 24 meses. As amostras de solo, para além da sua caracterização, foram submetidas a incubações aeróbias e anaeróbias em ambiente controlado, para estudar as dinâmicas do azoto e do carbono. Quantificaram-se as taxas de amonificação, nitrificação e mineralização líquida, bem como a imobilização do azoto na biomassa microbiana. Relativamente ao carbono, quantificou-se a mineralização através da actividade respiratória bem como a imobilização na biomassa microbiana. Avaliou-se também, o crescimento e estado nutricional das árvores. Concluiu-se que a aplicação de cinza beneficiou mais as características do solo que a adubação clássica, embora tal não se tenha reflectido no desenvolvimento das plantas. - EXTENDED ABSTRACT - The area occupied by Pinus pinaster Ait. stands in Portugal nowadays amounts to about 38% of the national territory, and after the cork oak this is the second more representative species of Portuguese forest. It is distributed across wide areas, and mainly located in the coastal regions in central and northern Portugal mainland. Most of maritime pine stands are located in areas with acidic and low fertility soils. Recently, there has been a considerable increase in waste production (sludge and wood ash) from the paper industry, and now there is a strong pressure on Portuguese Government and on the European Community, to approve legislation that encourage the reuse or treatment of industrial wastes in order to preserve the environment. Considering this problem, with this research it is intended to assess the effects of wood ash from forest biomass, on plants and soil, as an alternative to conventional industrial fertilizers (nitrogen and phosphate fertilizers), when applied to soil during the installation of a Pinus pinaster stand. The experimental plots were planted in March 2005 with a 4 m x 2 m spacing, in the region of Pedrógão Grande (Central Portugal) and it was used an experimental design of randomized blocks, comprising three blocks and five treatments. The five treatments were the following: CI - application of wood ash; CIN - application of wood ash and nitrogen fertilizer; FP - application of phosphorus fertilizer; FPN - application of phosphorus and nitrogen fertilizer; CT - control, without any ash or fertilizer application. Thus, soil samples were collected before planting and after 24 months to determine the effect of treatments in soil chemical characteristics. Those samples were analyzed in laboratory for the following parameters: pH, carbon and nitrogen concentrations, exchangeable base cations, and extractable phosphorus and potassium concentrations. In addition, from another soil sampling 24 months after planting, the carbon and nitrogen dynamics were studied, under controlled conditions. Two incubation methods were used to study nitrogen dynamics in laboratory: one under aerobic and another under anaerobic conditions. The aerobic incubation lasted 90 days, and sub-samples were taken after 0, 7, 15, 30, 60 and 90 days, in order to quantify ammonification, nitrification and net mineralization rates. For the same periods,a fumigation-extraction method(with chloroform) was used to evaluate nitrogen retention in microbial biomass. Nitrogen mineralization rate in anaerobic conditions was quantified on samples after an incubation period of 14 days. The study of carbon dynamics was held in parallel to nitrogen dynamics, on the same samples submitted to aerobic incubation. Carbon mineralization was determined by quantifying the release of carbon dioxide after 4, 7, 15, 22, 45, 60 and 90 incubation days. Carbon immobilization in microbial biomass was quantified in the same sub-samples used to evaluate nitrogen immobilization. Nutritional status of trees was also assessed through the collection and chemical analysis of needles, harvested 24 months after the application of different treatments. Growing conditions of the stand were finally characterized, evaluating mortality rates and measuring trees heights. From the obtained data, we may conclude that the application of wood ash led to an increase of soil pH, of extractable phosphorus and potassium, and of calcium and magnesium levels. This confirms the interest in using wood ash, as alternative alternative liming materials to agriculture limestone, in order to correct soil acidity and to improve soil fertility. Wood ash also leads to an increase of initial rates of nitrogen mineralization in soil, observed both under aerobic and anaerobic conditions, which however were decreasing over the incubation period. In addition, an increase of carbon dioxide release rate, especially during the initial period of incubation, was also reported, indicating that the application of wood ash have also contributed to a remarkable change in soil microbial population, and to an intensification of activity of these microorganisms. Another consequence of this intensification of microbial activity was, certainly, the increasing of competition for nutrients between plants and microorganisms. In fact, plots with ash application revealed lower tree growth than control ones, at least at the end of the short period of study (2 years). The application of industrial classic fertilizers, rich in phosphorus and nitrogen, was not reflected in any improvement of soil chemical characteristics or tree vitality, which were not significantly different from the control plots. In the same way, patterns of variation of nitrogen and carbon dynamics, in those treatments (FP and FPN), were very similar to those observed in the control. Then, we may conclude that, for the amounts considered in this study, the classic fertilization was irrelevant