『Abstract
A refined version of a conditional phosphorus risk index (PRI)
for P losses to waters was developed based on monitoring and analyses
of PRI factors from an agricultural catchment in Sweden. The catchment
has a hummocky landscape of heavy glacial till overlying moraine
and an overall balanced soil P level. Single P source factors
and combinations of factors were tested and discussed together
with water movement and water management factors important for
catchments dominated by drained clay soils. An empirical relationship
was established (Parson correlation coefficient 0.861, p＜0.001)
between phosphorus sorption index (PSI-CaCl2),
measured in a weak calcium chloride solution, and iron (Fe-AL)
aluminium (Al-AL) and phosphorus (P-AL) in soil extract with acid
ammonium lactate. Differing relationships were found for a field
that had not received any manure in the last 15 years and a field
that had received chicken litter very recently. In addition, a
general relationship (Pearson correlation coefficient 0.839, p＜0.001)
was found between the ratio of phosphorus extracted from fresh
soil in water (Pw) to PSI-CaCl2 and the degree
of phosphorus saturation in lactate extract (DPS-AL). One exception
was a single field, representing 7％ of agricultural land in the
catchment, that had been treated with glyphosate shortly before
soil sampling. Saturated hydraulic conductivity (SHC) in heavy
clay in contact with the moraine base (at 1 m depth) was on average
0.06 m day-1. In clay not in contact with moraine,
SHC was significantly lower (mean 0.007 m day-1). A
reduction in the present tile drain spacing (from 14-16 m to 11
m) is theoretically required to maintain satisfactory water discharge
and groundwater level. Up to 10％ of the arable land was estimated
to be a potential source area for P, based on different indices.
Parts of a few fields close to farm buildings (1％ of total arable
land) were identified as essential P source areas, with high DPS-AL
values and low PSI-CaCl2 values throughout
the soil profile. A further 2％ of arable land was identified as
potential important transport areas, based on visible surface
water rills or frequent water-ponded conditions. Fields comprising
10％ of the total arable land in the catchment should be re-drained
in the near future to improve water infiltration and avoid unnecessary
channelised water flow. The need for an improved PRI for erosion
and water transport is discussed.
Keywords: Degree of phosphorus saturation (DPS); Phosphorus risk
index (PRI); Phosphorus sorption index (PSI); Phosphorus water
extraction (Pw); Saturated hydraulic conductivity (SHC)』
1. Introduction
2. Materials and methods
2.1. Catchment and agricultural management
2.2. Soil sampling and analyses
2.3. Water sampling of single fields and water analysis
3. Results and discussion
3.1. Source factors
3.2. Erosion and structure index
3.3. Soil water retention and saturated hydraulic conductivity
3.4. Combining source and transport factors
3.5. Future implications
4. Conclusions
Acknowledgements
References