Fractions of organic matter and decomposition of eucalypt harvest residues in Coastal Plain soils of Bahia, Brazil / Frações da matéria orgânica e decomposição de resíduos da colheita de eucalipto em solos de Tabuleiros Costeiros da Bahia

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

Eucalypt is the most planted tree in Brazil and to maintain a sustainable forest production it is fundamental do understand the processes of litter decomposition and the impact of eucalypt cultivation on soil organic matter (SOM) fractions. The conversion of planted pastures in eucalypt plantations alters the labile and humified pools of SOM, but little is known about the effects of eucalypt plantations in those fractions in Argisols of Brazilian Coastal Plain. Thus, in the first two chapters of this work it was investigated the changes caused by eucalypt cultivation on carbon (C) and nitrogen (N) stocks in SOM fractions. The selected soils were sampled in the 0- 10, 10-20, 20-40, 40-60 and 60-100 cm depths in areas previously under planted pasture, and currently cultivated with short-rotation eucalypt at the end of the first rotation (implantation eucalypt); eucalypt in the second rotation, as a new stand (reformed eucalypt); planted pasture and, native vegetation (Atlantic Forest), used as a reference. The study was in a completely randomized block design with six replicates, and treatments analyzed as split-plot. The substitution of the native forest for planting pasture, and the implantation of the eucalypt in pasture land reduced total organic carbon (TOC), labile carbon (LC), free light fraction C (FLL) and humic substance (HS) C stocks. However, in the second rotation, when eucalypt was cultivated under reform, there was a recover in C stocks of those SOM fractions, which returned to values similar to those under the native forest soil. The pasture was the soil use that maintained the largest N stocks in all SOM fractions. Similarly to C, most of the N in the soil under pasture is associated to the humin fraction. The analysis of C in HS was sensitive to detect differences among the different soil uses, but the effects were more evident in the labile and intermediate SOM fractions. The soil microbial biomass (BMS) C stocks showed no significant difference among soil uses in surface layers, and significant differences were detected only when C stocks of deeper soil layers were taken into account. The adoption of reduced tillage, the on site debarking, and the increase of fertilization, especially with N, in plantation eucalypt is believed to produce plant residues of better quality and faster cycling rates. However, reports on the decomposition rate of different residue components and how it is influenced by their N content are scarce. In the third chapter, it is presented results on the dynamics of decomposition of individual eucalypt residue components (with and without bark) with distinct initial N content, under different climatic conditions in southern Bahia state, Brazil. The residues (leaves, branches) used in the decomposition study were collected in a nitrogen fertilization experiment, from three year-old clonal Eucalyptus grandis x E. urophylla trees not fertilized with N and from trees that were fertilized with high doses of N (320 kg ha-1). The bark used was from a nearby clonal 7,4 year-old eucalypt plantation that had been recently harvested with a harvester. Leaves, branches and barks were dried, weighed, combined and put inside litter bags, which were then were taken to the field. Each litter bag contained 40 g of residue. The litter bags were allocated in five regions (West, Central A, North, Central B and South) with increasing rainfall. Litter bags were collected at five periods (0, 1, 3, 6 and 12 months) after installation. The treatments consisted of two residue compositions (leaves + branches, with and without bark), two residue nutritional qualities (low and high N content), and five sampling periods (0, 1, 3, 6 and 12 months), in the five geographic regions with distinct rainfall. The experiment was in a completely randomized block design with five replicates (regions), and treatments arranged in a split-split plot design. The residues with higher initial N content and in regions with higher rainfall were more rapidly decomposed. The time required for 50 % of mass loss of the whole combined residue (leaf + branch + bark) (t0,5) varied from 248 to 388 days for residues with high initial N content, and from 322 to 459 days for residues with lower initial N content. However, with exception of the drier, West region, the presence of bark led to smaller decomposition constants (k) and larger t0,5 values for the combined residues that had greater initial N content. Of the individual components of the litter, the leaves were more rapidly decomposed. The decomposition of branches was stimulated by the presence of the bark, the greater initial N content and precipitation. Conversely, the bark that was in the presence of residues with greater initial content was less decomposed. The mass remaining of individual components of residue correlated negatively with N and lignin content. For branches and bark, the C:N and lignin:N ratios were important indicators of the resistance decomposition, because higher ration resulted in smaller mass loss and longer t0,5. The release of N from leaves followed the decomposition dynamics, while in more recalcitrant materials, with lower N initial content (branches and bark) it was observed a temporary net N immobilization.

ASSUNTO(S)

eucalyptus nitrogen soil organic matter ciencia do solo eucalipto nitrogênio matéria orgânica do solo

ACESSO AO ARTIGO

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