BENDING Gary D., TURNER Mary K.

Transcription

1 Scientific registration no : 2030 Symposium no : 9 Presentation : poster Effect of interaction between the biochemical and physical qualities of leaf and root materials on N dynamics following addition to soil Effet de l interaction entre les qualités biochimiques et physiques des litières épigées et racinaires sur la dynamique de N BENDING Gary D., TURNER Mary K. Department of Soil and Environment Sciences, Horticulture Research International, Wellesbourne, Warwickshire CV35 9EF U.K. Introduction Many studies have attempted to understand the relationships between the quality of plant materials as substrates for soil microbes, and rates of nutrient cycling processes. Much is known of the ways in which the biochemical composition of plant materials controls quality, and a range of biochemical quality parameters have been found to be correlated with decomposition and mineralization processes. Generally applicable biochemical quality components that have been correlated with these processes include C/N ratio, and nitrogen (N), lignin and cellulose contents (Iritani and Arnold, 1960; Frankenberger and Abdelmagid, 1985; Janzen and Kucey, 1988; Vigil and Kissel, 1991; Bending et al., 1998). The nature of biochemical quality components that control mineralization have been found to depend on the type of plant material being tested. For example, polyphenol content has been shown to be important in controlling N release from leguminous residues (Palm and Sanchez, 1991; Giller and Cadisch, 1997). Additionally, the importance of different quality parameters can change during the decomposition process. Bending et al., (1998) showed that for a range of leaf and root residues, early N mineralization was correlated with soluble-n content, while later release reflected cellulose content, total N and C/N ratio. Relatively little is known of the way in which physical quality affects mineralization. A diverse assemblage of properties contribute to physical quality. These include particle size, toughness, surface properties such as cuticle thickness, and the presence of defence structures such as spines (Swift et al., 1979). These properties have the potential to affect colonization of the substrate by soil organisms, and hence affect patterns of breakdown. The physical quality parameter that has received most attention in studies of decomposition and mineralization is particle size. Several studies have confirmed that particle size affects these processes, although the nature of the effect is variable, with some reports that reducing particle size increases rates of microbial processes (Sims and Frederick, 1970 Amato and Ladd 1984), while other investigations have shown the reverse effect (Stickler and Frederick, 1959; Jensen, 1994; Sorensen et al., 1996). There is a need to determine whether the contrasting results reflect the varied biochemical qualities of the plant materials used. The aim of this study was to investigate whether biochemical quality and particle size interact to 1

2 affect N dynamics during decomposition of plant materials. Fresh leaf and root materials representing a spectrum of biochemical qualities were cut into 2 particle sizes, representing high and low physical quality, and incorporated into soil. N dynamics were investigated over a period of 6 months. Materials and Methods Soil Soil was collected from the top 20 cm of a fallow field at Wellesbourne, Warwickshire, U.K. The soil is a sandy-loam of the Wick series, with 14 % clay, a ph of 5.9, an organic-c content of 0.8 %, and an organic-n content of 0.1 % (Whitfield, 1974). Soil was passed through a 2 mm sieve and air dried prior to use. Plant materials Leaf and root materials were chosen to represent a spectrum of qualities. The materials consisted of mature leaves and petioles of brussels sprouts (Brassica oleracea cv. Peer Gynt) and potato (Solanum tuberosum L. cv. Wilja), and roots of rye grass (Lolium perenne L cv. Parcour) all of which had been grown under glasshouse conditions in Levington M2 compost for up to 4 months. Prior to use, roots were washed thoroughly in deionized water to remove adhering soil. Wheat (Triticum aestivum L.) stubble was collected from a recently harvested field at Wellesbourne, air dried and stored at 4 o C before use. There were 2 particle size treatments for each residue, both of which were prepared by manually cutting the material with a pair of scissors. For the leaf treatments, the coarse and fine particle sizes consisted of 4 and 0.2 cm squares of leaf and petiole, while the rye roots and straw were cut into 4 and 0.2 cm lengths. Biochemical quality analysis Quality characteristics of the plant materials were determined by proximate analysis. Sub-samples of the tissues were dried at 100 o C and milled before investigation. A water-soluble fraction was removed by extracting residue with deionized H2O in a boiling water bath for 2 hours. The mixture was centrifuged, and acid-soluble and insoluble components were determined in the substrate remaining, as described by Bending et al. (1998). The material was subjected to hydrolysis with H2SO4 to convert cellulose to sugars, which were estimated in the acid hydrolyzable fraction using the phenol-h2so4 assay (Dubois et al., 1956). Acid insoluble material, consisting mainly of lignin, was determined by weight loss of the acid-insoluble fraction on ashing. Total residue organic-c was measured by dichromate oxidation (Anderson and Ingram (1991), and total residue organic-n by a micro-kjeldahl procedure (Anderson and Ingram, 1991). Incubation study Soil was moistened to a water potential of 320 kpa with deionized H2O, and incubated at 15 O C for 5 days prior to use. Replicate pots were set up by mixing 5, 1.5 and 1 g fw of the leaf, root and straw residues respectively into 100 g fw soil. The leaf and straw additions are typical of residue inputs to soil following cropping (Sylvester-Bradley, 1993), while the root addition represented the equivalent root biomass produced by the glasshouse grown plants. Pots were placed into sealed 5 L plastic chambers, through which moist air was passed continually to ensure that pots were incubated in an aerobic environment. For each residue and unamended soil, 3 pots were harvested after 3 d, at weekly intervals for the next month, and at monthly intervals for the following 3 months, with a final harvest 2 months later. At each harvest, soil mineral-n was extracted in 0.5 M K2SO4. NH4 + in the extract was measured by the indophenol blue assay (Scheiner, 1976) and NO3 - by High Performance Liquid Chromatography (Hunt and Seymour, 1985). Results were calculated by subtracting mineral N recorded in unamended soil. 2

3 Results Biochemical quality of plant materials The biochemical characteristics of the leaf and root tissues are shown in Table 1. Using the criteria of Bending et al. (1998) to define biochemical quality, the materials spanned a spectrum, ranging from high quality potato leaf, with low C/N ratio and cellulose content, and high amounts of N, to low quality straw, with high C/N ratio and cellulose content, and low amounts of N. Effect of biochemical and physical quality on soil nitrogen dynamics The effect of residue particle size on N dynamics depended on the biochemical quality of the residue (Fig 1a-d). Incorporation of potato leaf (highest quality), resulted in rapid net mineralization of N, with the major period of net mineralization occurring within the first 21 d following incorporation (Fig 1a). Particle size had no significant effect on N mineralization from this material. For lower biochemical quality brussels sprout leaf, net N mineralization was slower, and the major period of net mineralization occurred between 28 and 112 days. Particle size greatly affected mineralization from this material. Incorporation of fine particle size pieces resulted in a small peak of net N mineralization after 21 d, although this N was rapidly re-immobilized (Fig 1b). In contrast, during the first 28 d mineral-n was immobilized in soil receiving coarse particle size material. In both treatments net mineralization of N proceeded rapidly between 28 and 112 d, following which the soil mineral-n pool declined slightly. The rate of mineralization between 28 and 56 d was considerably higher in soil receiving coarse particles than fine particles, but there was little difference in the amount of N mineralized after 112 and 168 days. For low quality rye grass roots and straw, net immobilization of N occurred over the course of the experiment (Fig 1c,d). In the cases of both these materials, reducing particle size resulted in greater net immobilization of N, which increased over the course of the experiment). Discussion The results demonstrate that biochemical and physical qualities of plant materials interact to affect N dynamics following incorporation into soil. Particle size can affect the soil saprophyte community in several ways. Changing particle size of plant materials will alter the availability of cut sites for colonization by the saprophyte community. Particle size will also affect contact of the plant materials, microbial tissues and microbially synthesized organic materials with soil particles and components such as clays. This will provide stabilization and greater physical protection against further degradation. In addition, particle size will affect exchange of water and nutrients with the soil matrix, influencing microbial growth and activity. The relative importance of each of these effects is likely to vary depending on the biochemical quality of the plant material, soil type and the physical and chemical environment of the soil. In the case of high biochemical quality potato leaf, it is apparent that the substrate was so rapidly colonized by soil microbes that the availability of more sites for tissue colonization had no effect on colonization, or on subsequent decomposition and mineralization processes. The turnover of this substrate was evidently so rapid that there was no time for the effects of greater physical protection, afforded to the fine particles by more intimate contact with the soil matrix, to take effect. For lower biochemical quality brussels sprout leaf, reducing particle size and increasing available sites for colonization resulted in faster tissue colonization, leading to enhanced decomposition and mineralization immediately following addition to soil. However, the subsequent period of immobilization of N followed by slow net mineralization may reflect greater contact of the fine particles with the soil matrix. This would have resulted in greater physical protection of the residue, 3

4 microbial biomass and new microbially- synthesized organic compounds by clay particles. However, the fact that particle size had no effect on mineral-n produced by the end of the incubation suggests that such protection was short-lived. For the low biochemical quality materials, reducing particle size resulted in enhanced immobilization of soil mineral-n. During decomposition of such high C/N ratio materials, microbial growth is supported by utilization of soil mineral-n, and these results suggest that the increased availability of cut sites for tissue colonization in the fine particle size treatment resulted in more rapid microbial colonization and microbial growth. The continued enhanced immobilization of N by reducing particle size persisted throughout the incubation. This could reflect protection of microbial tissues and microbial products by soil particles, shielding them from subsequent degradation, and protecting N against subsequent re-use by the microbial community. Acknowledgements- We thank the Ministry of Agriculture, Fisheries and Food for financial support, and Julie Jones for statistical advice. References Amato M., Jackson R.B., Butler J.H.A. and Ladd J.N. (1984) Decomposition of plant material in Australian soils. II. Residual organic 14 C and 15 N from legume plant parts decomposing under field and laboratory conditions. Australian Journal of Soil Research 22, Anderson J.M.& Ingram J.S.I. (1991) Tropical soil biology and fertility: a handbook of methods. CAB International, Wallingford, UK. Bending G.D., Turner M.T. & Burns I.G. (1998) Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass. Soil Biology and Biochemistry (in review). Dubois M., Gilles K.A., Hamilton J.K., Rebers P.A. & Smith F. (1956) Colorimetric method for the determination of sugars. Analytical Chemistry 28, Frankenberger W.T. and Abdelmagid H.M. (1985) Kinetic parameters of nitrogen mineralisation rate of leguminous crops incorporated into soil. Plant and Soil 87, Giller K.E. and Cadisch G. (1997) Driven by Nature: A sense of arrival or departure? In Driven by Nature. Plant litter quality and decomposition. (G. Cadisch and K.E. Giller Eds), pp CAB International, Wallingford, UK. Hunt J. and Seymour D.J. (1985) Method for measuring nitrate-nitrogen in vegetables using anionexchange high-performance liquid-chromatography. Analyst 110, Iritani W.M. and Arnold C.Y. (1960) Nitrogen release from vegetable crop residues during incubation as related to their chemical composition. Soil Science 9, Janzen H.H. and Kucey R.M.N. (1988) C, N, and S mineralisation of crop residues as influenced by crop species and nutrient regime. Plant and Soil 106, Jensen E.S. (1994) Mineralization-immobilization of nitrogen in soil amended with low C:N ratio plant residues with different particle sizes. Soil Biology and Biochemistry 22, Palm C.A. and Sanchez P.A. (1991) Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic contents. Soil Biology and Biochemistry 23, Scheiner D. (1976) Determination of ammonia and Kjeldahl nitrogen by indophenol method. Water Research 10, Sims J.L. & Frederick L.R. (1970) Nitrogen immobilization and decomposition of corn residue in soil and sand as affected by particle size. Soil Science 109, Sørensen P., Ladd J.N. & Amato M. (1996) Microbial assimilation of 14 C of ground and unground plant materials decomposing in a loamy sand and a clay soil. Soil Biology and Biochemistry 28, 4

5 Stickler F.C. & Frederick L.R. (1959) Residue particle size as a factor in nitrate release from legume tops and roots. Agronomy Journal 51, Swift M.J., Heal O.W. & Anderson J.M. (1979) Decomposition in terrestrial ecosystems. Blackwell Scientific Publications, Oxford, 372pp. Sylvester-Bradley R. (1993) Scope for more efficient use of fertilizer nitrogen. Soil Use and Management 9, Vigil M.F. and Kissel D.E. (1991) Equations for estimating the amount of nitrogen mineralized from crop residues. Soil Science Society of America Journal 55, Whitfield W.A.D. (1974) The soils of the National Vegetable Research Station, Wellesbourne. Report of the National Vegetable Research Station for pp Keywords: biochemical quality, physical quality, nitrogen, mineralization, immobilization Mots clés : qualité biochimique, qualité physique, azote, minéralisation, immobilisation 5

6 Table 1 Biochemical quality characteristics of plant materials Plant material Composition (% dw) C/N N Acid-soluble fraction (Cellulose) Potato leaf Brussels sprout leaf Rye grass root Wheat straw Acid-insoluble fraction (Lignin) 6

7 Legend Figure 1 Soil nitrogen dynamics following incorporation of leaf and root materials into sandyloam. (, 4 cm pieces;, 0.2 cm pieces) Bars represent +/- standard error of the mean. Significance of difference between particle size treatments at each time interval determined by t-test (* significant p<0.05; ** significant p<0.01). 7

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