『Abstract
Concentrations of dissolved reactive phosphorus (CDRP)
and particle-bound phosphorus (CPP) were
investigated in soil columns taken from different parts of three
selected agricultural fields. The columns (mini-lysimeters 20
cm in diameter, 20 cm high) were exposed to on average 64±6 mm
of simulated rainfall. Total phosphorus concentrations (CTP) in water percolating from the lysimeters were
similar to the CTP observed in drainage water
leaving the fields, but the CDRP/CTP
ratio was significantly (p<0.001) higher in the percolate. The
high CDRP (up to 0.4 mg L-1) measured
in the percolate may have partly derived from decomposition of
accumulated senescing vegetation and litter material, since CDRP in percolating water was related to total
organic carbon concentration (TOC) in the shallow topsoil (0-5
cm) and to phosphorus concentration in soil extract of ammonium
lactate (P-AL) from the same soil layer. Another easily identified
factor clearly related to CDRP in percolate
from the lysimeters was the degree of phosphorus saturation (DPS)
in the same soil P-AL extract. The factors giving best prediction
of CPP and leaching were the amount of percolate
passing through the soil columns and total phosphorus concentration
in the topsoil determined after oxidation with nitric acid (TP-HNO3). One field under grass ley, including the slope
and depression close to the field outlet, comprised 24% of total
field area. In the other two fields these areas (11 and 17% of
total area respectively) were under long-term permanent green
fallow as an internal buffer zone (IBZ). The lysimeter studies
indicated that these areas had a higher risk of DRP losses than
the rest of the fields. In addition, the P content of the particles
in water from the depression was unusually high. Aggressive stability,
measured indirectly as readily dispersible clay and expressed
as nephelometric turbidity units (NTU), was significantly stronger
at the lowest point of one IBZ (27-35 NTU) than in stubbled parts
of the same field (62 NTU), and the turbidity of water percolating
through the topsoil was lower (5-27 NTU).
Keywords: Degree of phosphorus saturation; Phosphorus leaching;
Readily dispersible clay; Topsoil characteristics』
1. Introduction
2. Materials and methods
2.1. Agricultural fields and monitoring system
2.2. Sampling and soil analyses
2.3. Simulated rainfall and topsoil phosphorus leaching
2.4. Turbidity of percolate and dispersed clay from soil aggregates
3. Results
3.1. Phosphorus concentrations after simulated rainfall
3.2. Turbidity in percolate and as readily dispersed clay
4. Discussion
4.1. Risk of P leaching based on lysimeter studies
4.2. Turbidity as an indicator for soil dispersion risk
4.3. Importance of green fallow and grass for trapping and release
of DRP
5. Conclusions
Acknowledgements
References