Paula Muukkonen(1), Helinä Hartikainen(1), and Laura Alakukku(2, 3)
(1) Department of Applied Chemistry and Microbiology, Box 27, 00014 University of Helsinki, Finland, (2) MTT Agrifood Research Finland, E-House, 31600 Jokioinen, Finland, (3) Present address: Department of Agrotechnology, Box 28, 00014 University of Helsinki, Finland Paula.Muukkonen@helsinki.fi
Introduction
In Finland, almost 60 % of phosphorus (P) load originates from agriculture (Vuorenmaa et al., 2002). No-tillage, a method adopted to reduce erosion and particulate P load from fields to watercourses, is one of the several recommended methods to diminish the agricultural P load. However, in the no-tilled fields, where seeds and fertilizers are input straight to the soil without any tillage, P tends to accumulate in the uppermost soil layer. This increases the risk of P leaching in dissolved form with surface runoff (Puustinen et al., 2005). The objective of the present study was to examine the effects of tillage practice on the extractability and leaching of P from clay soil limed recently or 22 years ago.
Material and methods
The experimental material was collected in autumn from two clay-rich fields both of which had two no-tilled (for five years) and two ploughed plots. One of the fields was unlimed (limed 22 years ago), while the other field had been limed six months before soil sampling by mixing 7,000 kg of CaCO3 ha-1 into surface soil in connection with seedbed preparation. The unlimed field contained 46% clay (Eutric Cambisol) and the limed one 62% clay (Vertic Cambisol) in the soil layer of 0–20 cm.
For chemical analyses, we took the soil samples from the 0–2.5, 2.5–10 and 10–20 cm layers, four samples from each plot. The samples were air-dried, sieved through a 2 mm sieve and analyzed for dissolved reactive P (DRP) by shaking 1 g of soil in 50 ml of deionized water for 21 hours. The extracts were filtered through a 0.2 µm Nuclepore polycarbonate filter and the filtrate was analyzed for DRP by a
molybdenum blue method (Murphy & Riley, 1962). Soil organic carbon (OC) was determined with a LECO CN–2000 analyzer, soil pH in water suspension (1:2.5), and the easily soluble calcium (Ca) by shaking 5 g of soil in 250 ml ammonium acetate (pH 4.65).
Furthermore, we took four undisturbed soil columns from each plot for leaching tests.
A PVC-column (diameter 15 cm and height 20 cm) was hammered into the soil, plant residues were removed from the surfaces of the samples and the bottom surfaces of the soil columns were prepared with a knife and cleaned by vacuum to open the
pores. In the laboratory, the samples were saturated from the bottom upwards with deionized water for 1.5 hours and the percolation waters were collected over 0.5 h.
This treatment was repeated three times. Thereafter the 0–5 cm layers were removed from the columns, saturated with water that was then allowed to leach on same way as the 20-cm columns. The percolation waters were analyzed for DRP as above.
Results
We found that no-tillage had increased organic C in the surface soil layer of both fields (Table 1). In the unlimed field it had also slightly lowered pH of the surface soil, whereas in the ploughed plots pH remained almost the same in all layers studied (Table 1). In the limed field, the effect of liming was distinct in the surface soil layer, where pH and the concentration of Ca were clearly elevated irrespective of cultivation method. The concentration of Ca was notably higher in the limed field compared to the unlimed field. Interestingly, in the no-tilled plots, DRP had accumulated in the surface layer of the unlimed soil, but not in the limed one (Table 1).
Table 1. Soil organic C (OC), pHwater, easily soluble calcium (Ca) and dissolved reactive P (DRP) in various soil layers of unlimed and limed plots of fields cultivated with different tillage practices.
Unlimed field Limed field
OC pH Ca DRP OC pH Ca DRP
Layer (cm) (%) (mg kg-1) (mg kg-1) (%) (mg kg-1) (mg kg-1)
No-tillage 0–2.5 3.0 6.0 1900 35 3.4 6.8 5000 19
2.5–10 2.5 6.1 2200 24 2.8 6.1 2600 21
10–20 2.4 6.3 2400 23 2.7 6.2 2500 19
Ploughing 0–2.5 2.5 6.2 2200 22 2.7 7.0 3700 14
2.5–10 2.5 6.0 2200 23 2.7 6.5 3200 14
10–20 2.5 6.1 2200 23 2.6 6.2 2400 16
Regardless of the tillage method, the percolation waters from the soil columns taken from the limed field had on average less dissolved P than the waters from the columns taken from the unlimed field (Fig. 1). The same trend was found in the surface soil samples (0–5 cm) removed from the soil columns after the percolation test.
0,00 0,05 0,10 0,15 0,20
ploughing no-tillage ploughing no-tillage
unlimed limed
DRP (mg l-1 )
Figure 1. Dissolved reactive P (DRP) in the percolation waters from the undisturbed soil columns (height 20 cm). Each concentration is an average of three leaching events from eight soil profiles and the bars indicate the confidence interval of 90%.
Discussion
The switch from ploughing to no-tillage affects mainly the surface soil. When the soil is no longer ploughed, the residue of fertilizers and organic C easily accumulate in the uppermost soil layer, which can lower pH (Tarkalson et al., 2006, Muukkonen et al., 2007). Low pH further enhances the adsorption and saturation of P in the surface layer, and the risk of P leaching in surface runoff increases.
In this study, accumulation of P in the surface layer of the unlimed no-tilled plots was similar to the study of Muukkonen et al. (2007). However, the constant DRP
concentration in the whole 0–20 cm layer of the limed no-tilled plots indicates that the liming-induced increase in soil pH had enhanced the solubility of P and, thus, promoted its uptake by plants and/or movement downwards in the soil profile.
On the other hand, the high concentration of Ca in the limed soil may have diminished DRP in the percolation waters, because divalent cations enhance the adsorption of P on the soil particles (Barrow & Shaw, 1979). We suggest that liming, combined with increasing the ionic strength of the soil solution is an interesting option for reducing the losses of dissolved P with surface runoff. However, we need more information about the liming-induced net changes in DRP, because an increase in pH is known to promote the solubility of P. In the future, we would be interested to know whether liming increases the lability of soil P even though the soluble P may be lowered through the increase in ionic strength of soil solution.
References
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