Little is known regarding the effects of land use on soil organic matter and P
status of South African soils. For that reason, the effects of the main
agricultural land uses in the midlands region of KwaZulu-Natal [maize (Zea
mays), sugarcane (Saccharum spp), annual ryegrass pasture (Lolium
multiflorum), permanent kikuyu pasture (Pennisetum clandestnum), gum
(Eucalyptus grandis) and pine (Pinus patula)] on soil organic matter content,
microbial biomass C and P and inorganic and organic P pools derived from a
modified Hedley P fractionation was investigated on two sites where the longterm
history of land management was known.
In comparison with undisturbed native grassland, permanent kikuyu pasture
resulted in an increase in organic C, organic P and microbial biomass C and
P. Maize and sugarcane production resulted in a decrease in organic C,
organic P and microbial C and P. Under annual pasture, gum and pine
forests, organic matter and microbial biomass concentrations remained similar
to those under native grassland. Under native grassland, extractable organic
P accounted for 50% or more of the total P content of soils but under
agricultural management with regular applications of fertilizer P, there was an
increase in the percentage of total P present as inorganic P.
Agricultural management greatly affected the distribution of P among the
various inorganic and organic P fractions. Resin-Pi and NaHC03-Pi (the
potentially-available forms of Pi) showed similar trends with land use being
greatly elevated under kikuyu pasture at both sites and sugarcane and maize
at one site. This accumulated Pi was thought to have originated from recent
fertilizer applications and possibly recently mineralized organic P. Trends for
NaOH-Pi with land use differed greatly from those of the Resin- and NaHC03Pi
fractions. Concentrations were notably high under maize and sugarcane
production. Of the pools of soil organic P, the NaHC03-Po fraction was most
greatly affected by land use, being elevated under kikuyu and decreased
under maize and sugarcane. This supports the assertion that it is the
NaHC03-Po fraction that is the most labile soil organic P pool. It was concluded that land use greatly affects soil organic C and P status, soil
microbial biomass C and P contents, soil inorganic P concentrations and the
distribution of P among the various P fractions.
A short-term (8 weeks) laboratory incubation experiment was carried out to
compare the effects of inorganic (KH2P04) and organic (cattle manure, poultry
manure and maize crop residues) sources of P, applied at a rate equivalent to
30 kg P ha-1
, on soil inorganic and organic P fractions and the potential
availability of soil P. Additional treatments consisted of lime [Ca(OHhl at 5 ton
ha-1 and lime plus inorganic P. Applications of lime raised soil pH to a similar
extent after 1, 4 and 8 weeks incubation. After 8 weeks, a small increase in
soil pH was also noted for the cattle and poultry manure and maize residue
treatments. For the inorganic P fractions, substantial treatment effects were
observed only for the Resin-Pi fraction. The inorganic P source was more
effective than the organic ones at increasing Resin-Pi after 1 and 4 weeks
incubation and of the organic sources, cattle and poultry manure were more
effective than maize residues. Resin-Pi concentrations generally increased
between 1 and 4 weeks incubation but then declined rapidly between 4 and 8
weeks incubation. After 8 weeks incubation, treatment effects on Resin-Pi
were small. Concentrations of NaHC03-Pi, dilute HCI-Pi and concentrated
HCI-Pi all declined over the incubation period. There was no clear trend with
incubation for NaOH-Pi although for the poultry manure and maize
treatments, concentrations declined between 4 and 8 weeks incubation. In
general, concentrations of NaHC03-Po were greater for organic than
inorganic P sources after 8 weeks incubation suggesting microbial
immobilization of P in these treatments. There were increases in NaHC03-Po
and concentrated HCI-Po over the incubation period suggesting progressive
immobilization of P from the Pi fractions that declined in concentration during
the incubation. Concentrations of NaOH-Po were not greatly affected by
incubation period. The lime treatments, however, had lower NaOH-Po
concentrations than the others suggesting that liming may have stimulated
microbial mineralization of Po. Residual-P concentrations increased over the
incubation period. This was attributed to conversion of extractable Pi fractions
into recalcitrant, non-extractable Pi forms and/or immobilization of Pinto
intransigent organic forms. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/3457 |
| Date | January 2006 |
| Creators | Majaule, Ugele. |
| Contributors | Haynes, Richard J. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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