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AGGREGATE STABILITY AS AFFECTED BY SHORT AND LONG-TERM TILLAGE SYSTEMS... 767
AGGREGATE STABILITY AS AFFECTED BY SHORT
AND LONG-TERM TILLAGE SYSTEMS AND
NUTRIENT SOURCES OF A HAPLUDOX IN
SOUTHERN BRAZIL
Milton da Veiga (2)^ , Dalvan José Reinert (3)^ & José Miguel Reichert (3)
SUMMARY
The ability of a soil to keep its structure under the erosive action of water is usually high in natural conditions and decreases under frequent and intensive cultivation. The effect of five tillage systems (NT = no-till; CP = chisel plowing and one secondary disking; CT = primary and two secondary distings; CTb = CT with crop residue burning; and CTr = CT with removal of crop residues from the field), combined with five nutrient sources (C = control, no nutrient application; MF = mineral fertilizers according to technical recommendations for each crop; PL = 5 Mg ha-1^ y-1^ fresh matter of poultry litter; CM = 60 m^3 ha-1^ y-1^ slurry cattle manure; and SM = 40 m 3 ha -1^ y -1^ slurry swine manure) on wet-aggregate stability was determined after nine years (four sampled soil layers) and on five sampling dates in the 10 th^ year (two sampled soil layers) of the experiment. The size distribution of the air-dried aggregates was strongly affected by soil bulk density, and greater values of geometric mean diameter (GMD (^) AD) found in some soil tillage or layer may be partly due to the higher compaction degree. After nine years, the GMD (^) AD on the surface was greater in NT and CP compared to conventional tillage systems (CT, CTb and CTr), due to the higher organic matter content, as well as less soil mobilization. Aggregate stability in water, on the other hand, was affected by the low variation in previous gravimetric moisture of aggregates, which contributed to a high coefficient of variation of this attribute. The geometric mean diameter of water-stable aggregates (GMDWS ) was highest in the 0.00–0.05 m layer in the NT system, in the layers 0.05–0.10 and 0.12–0.17 m in the CT, and values were intermediate in CP. The stability index (SI) in the surface layers was greater in treatments where crop residues were kept in the field (NT, CP and CT), which is associated with soil organic matter content. No differences were found in the layer 0.27–0.32 m. The effect of nutrient sources on GMD (^) AD and GMD (^) WS was small and did not affect SI.
Index terms: soil tillage, manure, sampling time, soil structure, aggregation.
(1) (^) Part of the Thesys of the first author. Received for publication in march 2008 and aproved in a aplil 2009. (2) (^) Researcher of the Epagri, Experimental Station of Campos Novos, P.O. Box 116, CEP 89620-000 Campos Novos (SC). E-mail:
milveiga@epagri.sc.gov.br (3) (^) Professors of the Soils Department, Federal University of Santa Maria (UFSM), 97105-900, Santa Maria, RS, Brazil.
768 Milton da Veiga et al.
RESUMO : EFEITOS DE CURTO E DE LONGO PRAZO DA APLICAÇÃO DE
SISTEMAS DE MANEJO DO SOLO E DE FONTES DE
NUTRIENTES SOBRE A ESTABILIDADE DE AGREGADOS, EM
UM NITOSSOLO VERMELHO
A capacidade do solo em manter sua estrutura diante da ação de agentes erosivos como a água é geralmente alta em condições naturais e reduz quando o solo é sujeito a um preparo frequente e intensivo. O efeito de curto e de longo prazo de cinco sistemas de preparo do solo (PD = plantio direto; PE = escarificação e gradagem; PC = aração e duas gradagens; PCq = PC com resíduos queimados; e PCr = PC com resíduos retirados) associados com cinco fontes de nutrientes (T = testemunha; AM = adubação mineral de acordo com a recomendação para manutenção de cada cultura; EA = 5 Mg ha-1^ ano-1^ de cama de aviário, base úmida; EB = 60 m^3 ha-1^ ano-1^ de esterco líquido de bovinos; e 40 m^3 ha-1^ ano-1^ de esterco líquido de suínos) sobre a estabilidade dos agregados foi determinado ao final do nono ano de experimentação (coleta em quatro camadas) e em cinco épocas de amostragens efetuadas durante o décimo ano (coleta em duas camadas), em um Nitossolo Vermelho com alto teor de argila, no Sul do Brasil. A distribuição de tamanho de agregados secos ao ar foi fortemente alterada pela densidade do solo, e os maiores valores de diâmetro médio geométrico (DMGSA ) encontrado em alguns sistemas de preparo ou camadas podem ser parcialmente devidos ao maior estado de compactação do solo. Depois de nove anos, o PD e o PE apresentaram maior DMGSA na camada superficial, em comparação com os sistemas de preparo convencional (PC, PCq e PCr), devido ao maior conteúdo de matéria orgânica, bem como à menor mobilização do solo. A estabilidade dos agregados em água, por outro lado, foi influenciada pela pequena variação na umidade das amostras por ocasião da realização do teste, resultando em alto coeficiente de variação dessa determinação. O PD apresentou maior diâmetro médio geométrico dos agregados estáveis em água (DMGEA ) na camada de 0,00–0,05 m; o PC, nas camadas de 0,05–0,10 e 0,12–0,17 m; e o PE, valores intermediários. O índice de estabilidade dos agregados (IE) na camada superficial foi maior nos tratamentos em que os resíduos das culturas foram mantidos na lavoura (PD, PE e PC), o que está associado com o conteúdo de matéria orgânica. Não foram encontradas diferenças na camada de 0,27–0,32 m. As fontes de nutrientes mostraram pequeno efeito sobre o DMGSA e DMGEA e nenhum efeito sobre o IE.
Termos de indexação: preparo do solo, estercos, tempo de coleta, estrutura do solo, agregação.
INTRODUCTION
Soil structure has been defined as the size, shape and arrangement of the solid particles and voids, and is highly variable and associated with a complex set of interactions among mineralogical, chemical and biological factors (Letey, 1991). Although soil structure is not considered a factor directly related to crop production, it plays an important role in water and air supply to roots, root elongation, nutrient availability and macrofauna activity. A favorable structure for plant growth can be defined in terms of the presence of pores for water storage in a tension range available to crops, pores for water and air transmission and pores in which roots can grow (Oades, 1984).
For agriculture or horticulture, a soil should have not only a good structure, but also a structure which will persist for a long time, e.g., a structure of high quality and stability (Dexter, 1988). This author classifies structure stability in two principal types: (a) the ability of a soil to keep its structure under water action; and (b) the ability of moist soil to keep
its structure under the action of external mechanical stresses. The first type of structure stability is commonly evaluated through wet-sieving methods to determine aggregate stability in water, as proposed by Yoder (1936) and Kemper & Chepil (1965). The structure stability under external stresses can be determined in tests of compressibility (Gupta et al.,
- and shear strength (Fredlung & Vanapalli, 2002).
The best soil structure is usually found under natural conditions, and the structure of most soils under frequent and intensive cultivation is deteriorated. This can be measured by the decrease in aggregate stability (Carpenedo & Mielniczuk, 1990; Da Ros et al., 1997; Silva & Mielniczuk, 1997, 1998; D’Andréa et al., 2002). Among tillage systems, aggregate stability in the surface layer under no-till is usually greater than in conventional tillage systems (Hamblin, 1980; Carpenedo & Mielniczuk, 1990; Campos et al., 1995; Castro Filho et al., 1998; Beutler et al., 2001; D’Andréa et al., 2002), but in both soil aggregates may be compacted, with predominance of micropores (Carpenedo & Mielniczuk, 1990).
770 Milton da Veiga et al.
Aggregate stability was determined with the same cores used to establish the water retention curve. After application of 100 kPa tension, part of the sample was used to determine gravimetric water content and the remaining was carefully broken down to clods smaller than 0.008 m diameter, which passed through a 8.0 mm screen. The clods/aggregates were air-dried for 72 h under laboratory conditions and stored in metal cans shut with a lid (not hermetically closed), where they remained until the aggregate stability test was performed. These aggregates were labelled “air- dried aggregates”.
Air-dried aggregate size distribution
The air-dried aggregates (AD) were spread carefully in a plastic box using right-and-left movements, starting at one end and moving to the center of the box, in order to avoid segregation. A small plastic box with rectangular edges was used to sample approximately 25 to 30 g of air-dried aggregates all along the band of previous right-and-left disposal. At the same time, 10 to 15 g was sampled to determine gravimetric moisture.
For size distribution determination, a nest of sieves with opening screens of 4.0, 2.0, 1.0, 0.5, and 0.25 mm was used. Aggregates which passed through a sieve with 0.25 mm opening were collected at the bottom. The aggregate sample was spread on the upper sieve and the set was submitted to 12 gentle right-and-left movements, turned 90º and submitted again to 12 gentle right-and-left movements, to allow that only aggregates with a diameter greater than the respective open mesh of each sieve would be maintained on it, without applying excessive disruptive energy. The mass of aggregates retained on each sieve was used to calculate the AD geometric mean diameter (GMDAD) using the following equation:
where i denotes the aggregate classes (8.0-4.0; 4.0- 2.0; 2.0-1.0; 1.0–0.5; 0.5–0.25; and < 0.25 mm), ri the ratio of aggregate mass from class i related to the total, and d (^) i the mean diameter for class i.
Wet-aggregate size distribution and stability index
The aggregates from all sieves of the previous measurements were joined for wet-aggregate stability determination. For this measurement, a method similar to the modified approach of Kemper & Chepil (1965) was used. Nests of sieves with 4.0, 2.0, 1.0, 0.5 and 0.25 mm open mesh were placed within individual PVC tubes. The water level in each tube was enough to touch the bottom of the top sieve on the upper position of the apparatus. Aggregate sample was spread on the top sieve and allowed to saturate by capillarity during approximately 1 min, and then the water level was raised until the sample in the top
sieve was just covered. Samples remained in this condition for 10 min for complete wetting, and than apparatus was, applying up-and-down movements of approximately 4.0 cm throughout the water, at a frequency of 42 times per minute. After that, the nests of sieves were removed and the aggregates remaining on each sieve were transferred to individual cans, oven-dried and weighed, to determine the aggregate mass of each class. The mass of aggregates < 0.25 mm diameter was determined as the difference of total mass of aggregates (oven-dried mass) and the sum of oven-dried mass of aggregate classes
0.25 mm diameter. Since the sand content was low (< 3 %) in the Ap horizon, it was not removed to correct calculations of aggregate stability.
The water-stable geometric mean diameter (GMDWS) was determined using the same equation as described for size distribution of air-dried aggregates. Additionally, the aggregate stability index (SI) was determined by the relation between water- stable and air-dried mean diameter:
SI = GMDWS/GMDAD (2)
Chemical analysis
Chemical analysis was performed in disturbed samples collected at the same time as soil cores, from the layers 0.00–0.05, 0.05–0.10, 0.10–0.20 and 0.20– 0.40 m. Soil for chemical analysis was sampled at four positions in each plot, mixed, oven-dried at 60 °C during 48 h, ground with an electronic device and stored in paper boxes. The chemical analysis was performed at the Laboratory for Soil Analysis of the Epagri Research Center for Family Agriculture (Chapecó, SC), using the method described in CFSRS/ SC (1995).
Statistical analysis
The analysis of variance (ANOVA) test was run to quantify variances among tillage systems, nutrient sources, soil layer and sampling dates. Mean differences were compared using the Tukey test (p < 0.05). Due to the covariance between gravimetric moisture and aggregate stability, the procedure of general linear models was performed to determine mean differences among tillage systems in each layer, among layers within each tillage system, and among tillage systems across sampling times. The Pearson correlation was established among aggregate stability indexes and soil properties.
RESULTS AND DISCUSSION
Tillage systems showed differences for geometric mean diameter of water-stable aggregates (GMDWS) and stability index (SI) (Table 1). Differences among
AGGREGATE STABILITY AS AFFECTED BY SHORT AND LONG-TERM TILLAGE SYSTEMS... 771
tillage systems and sampling layers and SI were observed in the dry (GMDAD) and water-stable size distributions, with interaction between these two sources of variation at the end of the ninth year of treatment application. The differences among nutrient sources were smaller, and there was no interaction with tillage systems and sampling layers. Aggregates sampled on five dates during the 10 th^ year showed statistical differences among tillage systems for GMDws, and SI among sampling dates, layers, with interactions between sampling dates and layers or tillage systems for some parameters (Table 2). In both studies, the coefficient of variation was high, especially for GMDWS and SI, which can be partly explained by the variation in gravimetric moisture of aggregates at the time of aggregate stability determination.
Gravimetric moisture correlated best with aggregate stability in water, both for GMDWS and SI, in soil sampled after nine years of treatment (Table 3).
When dry aggregates were allowed to saturate in contact with water at atmospheric pressure, air bubbles are entrapped inside the aggregate and are compressed by water pulled into it by capillarity, until the air bubbles burst out of the partially wetted aggregate, with its partial disintegration (Kemper & Koch, 1966). The wetter the aggregate, the smaller the effect of air bubble entrapment, which may be expressed in a high positive correlation between gravimetric moisture and aggregate stability (Table 3 and Figure 1). The size distribution of air-dried aggregates (GMDAD), on the other hand, was strongly affected by bulk density, determined in the same core as used to sample aggregates (Table 3 and Figure 2). Since almost the whole volume of the soil core sampled was used in this determination, the greater the bulk density, the greater the mean aggregate diameter obtained by the disruption of the soil core. This correlation explains part of the differences found in
Table 2. Analysis of variance (ANOVA) for size distribution and aggregate stability indexes, determined in two layers on five sampling dates in the tenth year under five tillage systems and five mineral nutrient sources
U: gravimetric moisture; GMDAD: geometric mean diameter of air-dried aggregates; GMD (^) WS: geometric mean diameter of water- stable aggregates; and SI: aggregate stability index. ***, **, *, and ns: statistical significance at 0.1, 1 and 5 % and non- significant.
Table 1. Analysis of variance (ANOVA) for size distribution and aggregate stability indexes, determined in four layers after nine years under five tillage systems and five nutrient sources
U: gravimetric moisture; GMDAD: geometric mean diameter of air-dried aggregates; GMD (^) WS: geometric mean diameter of water- stable aggregates; and SI: stability index of aggregates. ***, **, *, and ns: statistical significance at 0.1, 1 and 5 % and non- significant.
AGGREGATE STABILITY AS AFFECTED BY SHORT AND LONG-TERM TILLAGE SYSTEMS... 773
hand, promotes aggregate stabilization because organic binding agents may have a transient, temporary or persistent effect, depending on which binding agent is involved in stabilization (Tisdall & Oades, 1982). The correlation between pH and available phosphorus with aggregate stability is probably due to the association of these parameters with others involved in aggregate formation and stabilization.
The geometric mean diameter of air-dried aggregates was greater in the intermediate soil layer and lower in the surface layer (Table 4). In the intermediate layer, the bulk density was greater (data not shown), showing close association between both parameter (Table 3 and Figure 2). Differences among tillage systems were found in the layers 0.00–0. and 0.12–0.17 m. After nine years of tillage system,
the GMDAD in the NT and CP was higher in the 0.00– 0.05 m layer, but no differences were found in the surface layer (0.025–0.075 m layer) when considering all five sampling dates performed during the tenth year (Table 5). The higher values in the surface layer of NT and CP can be explained not only by the higher organic matter content (Table 6), but also because the soil is less broken up by soil till than in the conventional systems (CT, CTb and CTr). This statement can be confirmed by the increase in GMDAD 240 and 360 days after sowing (Table 7), when natural soil reconsolidation occurred, resulting in increased bulk density. There were differences in GMDWS among tillage systems and layers (Table 4). Since this determination was highly correlated with aggregate moisture at the
Table 4. Size distribution and aggregate stability indexes in four layers, after nine years under five tillage systems (averaged across nutrient sources)
NT: no-till; CP: chisel plowing; CT: conventional tillage; CTb: CT with burning of crop residues; and CTr: CT with removal of crop residues. Means followed by the same lower case letters in a given row and capital letters in a given column do not differ statistically (Tukey, p < 0.05).
774 Milton da Veiga et al.
time of aggregate stability analysis (Table 3 and Figure 1), differences among means were also compared with least square means, using general linear models and considering covariance among these variables, both for tillage system within each layer and for layer within each tillage system. This procedure improved the mean differentiation among soil tillage in each layer, despite the low variability in aggregate moisture within each one, and corrected mean values for soil layers, where the variation in aggregate moisture was greater due to variation in storage time among them (data not shown).
Greater differentiation in GMD (^) WS among tillage systems was found in the surface layer (Tables 4 and 5). NT system showed greater GMDWS in the 0.00– 0.05 m layer, CT in 0.05–0.10 and 0.12–0.17 m layer, and CP intermediate values. A similar trend as in 0.00–0.05 m was observed in the 0.025–0.075 m layer when soil cores were sampled on five dates during the tenth year of treatments (Table 5). These results can be explained by the differences among tillage systems concerning crop residue disposition after tillage (on the surface in NT, partly incorporated in CP and incorporated in the arable layer in CT) and,
Table 5. Bulk density, gravimetric moisture at time of water aggregate stability test, size distribution and aggregate stability indexes at two layers and in five tillage systems (averaged across sampling dates performed during the tenth year)
NT: no-till; CP: chisel plowing; CT: conventional tillage; CTb: CT with crop residues burned; and CTr: CT with crop residues removed; GMDAD: geometric mean diameter of air-dried aggregates; GMD (^) WS: geometric mean diameter of water-stable aggregates; GMD (^) WSC : geometric mean diameter of water-stable aggregates corrected to aggregate moisture; SI: aggregate stability index; SI (^) C: aggregate stability index, corrected to aggregate moisture; U: gravimetric moisture; and BD: bulk density. Means followed by the same lower case letters in a given row do not differ statistically (Tukey, p < 0.05).
Table 6. Organic matter content in four layers after nine years under five tillage systems (averaged across nutrient sources)
NT: no-till; CP: chisel plowing; CT: conventional tillage; CTb: CT with burning of crop residues; and CTr: CT with removal of crop residues.
776 Milton da Veiga et al.
sources on size distribution and aggregate stability can be associated to the low rates applied (only for nutrient supply) and high soil clay content, since the effect of organic matter is more pronounced in soils with higher application of organic material (Weil & Kroontje, 1979) or containing smaller amounts of the finer particle class (Baver et al., 1972).
CONCLUSIONS
- Air-dried aggregate size distribution is strongly affected by bulk density, and greater values of geometric mean diameter are found in no-till and chisel plow systems, or in compacted layers under conventional tillage system. The stability index is highly correlated with previous gravimetric aggregate moisture. In treatments where crop residues are maintained in the field, the stability index in the surface layer is greater, which is associated with organic matter supply. Soil tillage did not affect aggregation in the 0.27–0.32 m layer.
- The geometric mean diameter of water-stable aggregates in the no-till system was highest in the 0.00–0.05 m layer, lowest in the 0.05–0.10 and 0.12– 0.17 m layers under conventional tillage, and intermediate under chisel plowing. The effect of nutrient sources on aggregate stability is lower than of tillage systems after nine years of annual application.
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