Little is known about the effects of land use on soil organic matter and P status of South African soils. Effects of long-term land use at Site 2 on the proportions of P present as labile, moderately labile P and stable P 56. This project focuses on aspects of soil P dynamics affected by long-term land use and management practices.
Introduction
Soil phosphorus pools
- Active P
- Fixed P
Inorganic phosphorus compounds present in the soil are orthophosphate (H2P04- and HPO/- and P043-), calcium phosphate, aluminum and iron oxide phosphates (Cade-Menum et al., 2000). The Po content of soils generally increases with increasing organic C and/or N (Tisdale et al., 1985). Phytic acid is an orthophosphate monoester that is biologically stable in soils (Tisdale et al., 1985).
Soil microbial biomass
Mineralization of organic P is determined by its chemical nature and associated reactivity (Condron et al., 1985; Turner et al., 2003). A decrease in soil N or P can result in increased mineralization of the other (Tisdale et al., 1985). Determining organic P mineralization is difficult because inorganic P (Pi) released from mineralization can be removed from solution by (1) P retention in clay and other soil surfaces, and (2) P precipitation as secondary minerals AI3+, Fe2+ or Ca2+_p (Tisdale et al., 1985).
Hedley fractionation method
- Resin - P
- Hydrochloric acid - P (HeI - P)
- Residual - P
When the P fraction of the resin decreases, phosphatase enzyme activity may increase, with a corresponding increase in P mineralization of organic P (Schmidt et al., 1996). The bicarbonate Po fraction consists of compounds such as ribonucleic acids and glycerophosphate and can be assimilated by plants (Hedley et al., 1982; Buehler et al., 2002). NaHCO3 - P is considered to be labile Po sorbed onto soil surfaces plus a small amount of microbial P (Schmidt et al., 1996; Zhang and MacKenzie, 1997).
Sodium hydroxide extractable Pi is assumed to be non-occluded P associated with amorphous and some crystalline AI and Fe hydroxides (Fe- and AI-bound Pi) (Beck and Sanchez, 1994; Buehler et al., 2002). In acid soils, this P fraction can act as a sink for P during excess P fertilization, but is also a P source when resin P levels are low (Hedley et al., 1982). In highly weathered P-deficient soils, the hydroxide-extractable Pi fraction can be significantly increased by P additions because the high exchangeable KI in these soils (Buehler et al., 2002) results in precipitation of KI phosphates.
These organic materials are reported to be stable and involved in long-term soil P transformations (Guppy et al., 2005). Some workers have identified NaOH-extractable Po as an important source of P in unfertilized tropical soils (Tiessen et al., 1982; Schmidt et al., 1996). HCI - Pi is relatively stable and bound to Ca (Ca-P) and can be associated with apatite minerals (Lehmann et al., 2001).
Figure 2.9 Modified Hedley sequential fractionation procedure for soil phosphorus (Pi, Po and Pt refer to inorganic, organic and total P respectively) (Source: . O'halloran et al., 1987).
Effects of soil management on P fractions
- Arable systems and tillage methods
- Pasture systems
- Organic amendments
As a result, there is increased soil organic P accumulation while P fertilizer applications tend to increase Pi levels. In some cases, where soil organic matter accumulates, there is also an accumulation of P in the Po fractions. Application of inorganic P fertilizer was found by O'halloran et al. 1987) to result in significant increases in some labile P fractions (resin- and NaHC03 - Pi), but to have no effect on NaHC03 - and NaOH- Po in a brown chernozem fertile soil.
Where fertilizer P input exceeded crop uptake, they also observed P accumulation in the NaHC0. The importance of soil texture for soil P distribution was confirmed by Tiessen et al. They noted that in cultivated soils with limited P input, the decrease in P status was mainly characterized by decrease in the inorganic P fraction in a coarse-textured soil, while in fine-textured soil the decrease was exclusively related to decrease in organic P fraction. .
Increased P availability as a result of the addition of organic fertilizers has been widely reported (Iyamuremye et al., 1996; Zhang and MacKenzie, 1997). Several mechanisms have been implicated in the increase in resin extractable P by organic amendments; (1) mineralization of the residues releases inorganic and labile organic P (Tisdale et al. during decomposition processes, soil microorganisms release organic molecules which can form complexes with soil solution AI and Fe and thereby reduce its solubility, (3) adsorption of AI on organic residue surfaces, and (4) precipitation of AI by hydroxyl (OH-) ions from redox and ligand exchange reactions (Hue, 1992) Fertilizer addition often results in accumulation of soil organic matter and thus an accumulation of organic P (O' halloran, 1993).
As they increased incubation time, the concentrated HCI-Po and residual P increased significantly at the expense of NaOH-Po and dilute HCI-Pj • In a related study, Hedley et al. 1982) also reported increases in residual P with incubation time.
Soil test extractants for P
This trend can be explained considering that the hydroxide-extracted Pi (NaOH-Pj) is bound to the Fe and AI components of soil surfaces. However, Pi and Po fractions not measured by these extraction methods contribute to dissolved P levels and are affected by various soil amendments (Anderson, 1980; Richardset al., 1995). The extraction procedure promotes soil P desorption by increasing the activity of Fe and AI and the formation of FeF4- and AIF4- (Chang and Jackson, 1957).
Resorption of soluble P in colloids is reduced by the action of F in the extracting solution (Bray and Kurtz, 1945). Under calcareous conditions, CaC03 neutralizes the acid in the extraction solution and CaF2 formed by the reaction between Ca2. The reaction of this extractant in soil is similar to the Bray I method in that F in the extractant solution forms AIF4-, allowing the release of aluminum-bound phosphates into solution.
The acetic acid in Mehlich III reagent reacts with calcium phosphates in the soil to remove P, but is less aggressive than equivalent mineral acids. The release of soluble P during the decomposition of organic matter saturates the adsorption sites so that the added P fertilizer is taken up by plants. Long-term applications of fertilizers and manure increase the accumulation of labile Pi in surface soils (McKenzie et al., 1992; Beck and Sanchez, 1994; Buehler et al., 2002).
Previous researchers (Dominy et al., 2001; Dominy and Haynes, 2002) have identified sites with long histories that represent the main agricultural land use in the Midlands of KwaZulu-Natal.
Introduction
Development of appropriate P management strategies will thus require a good understanding of the effects of land use and management practices on soil P pools. Microbial biomass C and P content generally showed a similar trend with land use for organic C, and at both sites permanent kikuyu pastures had the highest microbial biomass (Figures 3.1 and 3.2). For Resin-Pi and NaHCO3-Pi, kikuyu pastures and sugarcane clearly had the largest values, and values for maize were also elevated over the other land uses.
For NaOH-Pi, land use trends differed greatly from those for Resin-Pi and NaHCO3-Pi. That is, the highest values were observed for maize, although values for kikuyu pastures and sugarcane were still higher than those for the other land uses. Differences due to land use were less pronounced for concentrated HCl-Pi than for the other Pi fractions.
Compared to other land uses in Baynesfield, labile organic P (NaHCO3-Po) was very high under kikuyu meadows (Figure 3.4a). For concentrated HCI-Po, variations due to land use were not large, with the highest values recorded for perennial ryegrass and the lowest values for maize. In Cedara, Resin-Pi and NaHCO3-Pi were clearly highest under kikuyu meadows and were not strongly influenced by other land uses (Figure 3.5a and b).
Labile P [defined as readily extractable Pi (Resin-Pi plus NaHCO3-Pi) plus readily mineralizable Po (NaHCO3-Po)] was greatest under kikuyu grazing in both.
Baynesfield)
Cedara)
D~~DDD
Effects of soil amendments on stable P (residual P)
The residual P fraction was the largest fraction in all incubated soils and increased over the incubation period (Appendix 4.1). The increase in residual P with increasing incubation time in both the control and treated soil may be related to precipitation of insoluble P compounds, reactions of adsorbed P through which it is bound to soil components and immobilization of P in organic forms that are not easy . extractable with resin, NaHCO3, NaOH or HCI.
Conclusions
The conversion of native grassland ecosystems to a long-term monoculture of maize and sugarcane has resulted in significant reductions in soil organic matter (SOM). Greater input of substrate and nutrients to the topsoil in no-tillage due to surface retention of crop and weed residues will contribute to higher microbial biomass compared to conventionally tilled soil. Decades of mineral P fertilization of agricultural soils increased the total extractable P in the surface horizon (0-5 cm).
There was also a change in the distribution and amounts of P within inorganic (Pi) and organic P (Po) pools. Accumulation of organic matter in the surface layer of the soil below ZT results in an accumulation of Po in the soil. The incubation experiment showed that applications of P from various organic and inorganic sources can have significant short-term effects on the P fractions.
However, in the long term, P sorption may reduce the availability of P derived from commercial fertilizers more than the availability of P derived from organic residues. Furthermore, the effects of soil amendments on P fractions can be assessed under field conditions, in the presence of crops. Although increases in soil P fractions were observed in the incubation experiment, it was uncertain whether the increases were due to transformation of natural P between pools or were due to applied P.
Isotopic labeling of added P has been used very effectively in tracing soil P pathways, and this could be a fruitful area where this research could be extended in the future.
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Long-term effects of fertilizer and manure application on soil phosphorus forms and availability.
