Qualidade e decomposiÃÃo de materiais orgÃnicos presentes em propriedades rurais do semi-Ãrido / Quality and decomposition os organic materials found in farms of the semi-arid region of ne Brazil / Qualidade e decomposiÃÃo de materiais orgÃnicos presentes em propriedades rurais do semi-Ãrido / Quality and decomposition os organic materials found in farms of the semi-arid region of ne Brazil

Romildo Nicolau Alves

Owners of small farms in the semi-arid region of NE Brazil usually do not use chemical fertilizers to ameliorate soil fertility. Therefore, the nutrients supply for their crops have to rely on soil organic matter mineralization or, in some cases, on the fertilization with animal manure (mostly from cattle or goats). However, the manure produced in these farms is usually of low quality, and may cause nutrient immobilization during the initial phase of decomposition. Recent studies have shown that tree prunings used as green manure could be a viable alternative for soil fertilization. The decomposition of plant biomass used as green manure, though, depends on several factors, such as the quality of the biomass, which is defined mainly by the contents of nitrogen (N), lignin (L) and polyphenols (PP), and also on other environmental and management factors, such as temperature, moisture and the way the materials are placed on the soil. The present thesis was divided in three studies, and had as general objective to evaluate the interrelationships between plant quality, management and N mineralization when using different plant species commonly found in farms of the semi-arid region of NE Brazil as green manure to fertilize cropping fields. In the first study, 24 plant species were collected and analyzed to determine the contents of N, L and PP. The biomass from these species was incorporated to an Entisol, incubated for 28 days, and the mineralization of N during this period was quantified. The contents of N, L and PP and their ratios (L/N, PP/N e L+PP/N) were used in a Principal Component analysis, where the materials were grouped accordingly to their similarities, having the N mineralized after 28 days of incubation as the response variable of interest. After the analysis, four groups were formed, but only one of them presented a coherent behavior, i.e., the materials in this group immobilized N and had low N and high PP contents. Low correlation coefficients were observed between N mineralized and the variables used to define biomass quality, both after 3 and 28 days of incubation. The biomass of some of the plant species, after 28 days of incubation, led to the mineralization of a high proportion of the N applied, while other species led to N immobilization. However, the results obtained did not allow us to clearly predict N mineralization based on the quality parameters evaluated. Due to this lack of response, a second study was conducted with the objective of evaluating the soil incubation methodology adopted, particularly to investigate the interrelationships between the quality, the size in which the samples of the plant species were fragmented and the N mineralization after incorporation to the soil. For this, samples from plant species of different qualities were fragmented in different sizes (1 and 5 mm) and incubated in an Entisol during 28 days. During the incubation period, soil mineral N was determined after 3, 7, 14 and 28 days, and regression equatios were adjuted to model the results. We found that materials with high N and low L and PP contents mineralized N during the whole incubation period, idepedent of the size in which the samples were fragmented. Some materials, when incorporated in the smaller size (1 mm) presented higher N immobilization. In general, we concluded that the size of the particles had an influence on the intensity but did not change the direction of the mineralization/immobilization processes during the incubations, independent of the quality of the materials. The third study was developed under field conditions, in the Cariri region of the state of ParaÃba, in the municipality of TaperoÃ. The objective of this study was to evaluate the effects of the type of placement (surface applied or incorporated to the soil) of organic fertilizers with different qualities on the availability of water and N in the soil and on the productivity of grain and straw by maize. The experiment was established in a randomized blocks design, with the treatments arranged in a 4 x 2 factorial with four fertilization treatments (Gliricidia sepium, Croton sonderianus, cattle e control) and two types of placement (surface applied or incorporated). The experiment was carried for two consecutive years (2007 and 2008), with an application of 15 t ha-1 of fresh matter of the manures in each year. G. sepium, which was the material with the highest quality (hign N and low L and PP contents), decomposed faster when incorporated than when surface applied. On the other hand, C. sonderianus, probably due to the high PP content, decomposed more slowly and preserved the soil moisture for a longer period when was surface applied in 2008. Overall, when evaluating the results of N mineralization and of maize productivity, we suggest additional studies are needed to test if the split application of the manures may be a more beneficial management practice to increase crop productivity in our study region.

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