1. Interactions between the phosphorus content of animal manures
and losses of phosphorus to water
S.M. O’Rourke(1), R.H. Foy(2), C.J. Watson(2) and C. Ferris(3)
(1) Department of Agricultural and Environmental Science, Queens’s University of Belfast, BT9 5PX, U.K.
(2) Agri-Environmental Branch, Agri-Food and Biosciences Institute, Newforge Lane, BT9 5PX, U.K.
(3) Agricultural Branch, Agri-Food and Biosciences Institute, Hillsborough, BT26 6DR, U.K.
Introduction
Feeding imported concentrates in Irish dairy systems results in phosphorus (P) surpluses that contribute to elevated levels of soil P as well as increased amounts of P in manures. However manipulation of concentrate composition can lower P content of diets without compromising animal performance (Ferris et al., 2005) and is therefore a mitigation option for P-sensitive catchments. Rainfall simulation studies conducted over the summer, winter and spring seasons of investigated how varying the P content of cattle manure applied to grassland impacted on P concentrations in surface runoff.
Figure 1: Mean TP concentrations in runoff during simulated events in summer, autumn and spring
Compared to the summer and spring experiments the application of manure in the winter experiment was followed by substantially larger concentrations of TP in runoff as higher antecedent soil moistures increased the volume of runoff generated enabling more P to be removed. Despite this difference, patterns of TP loss were similar for all seasons, with concentrations declining markedly with time. Soil moisture may not be the dominant factor controlling runoff volumes (Figure 2). In the summer and spring periods there was a significant increase in the TP concentration from the runoff of the control treatment suggesting substantial natural variation in runoff and a reflection of soil P mineralization in the growing seasons.
Method
Holstein dairy cows fed four levels of dietary P from a 50:50 ratio of grass silage to concentrates, produced slurry P contents of 1.3, 1.0, 0.9 and 0.5 % P DM. Grassland runoff plots were laid out in a Latin square design. Following surface applications of manure at a rate of 50m3 Ha-1 and 6% DM, rainfall simulations using a portable rainfall simulator of the Amsterdam design (Bowyer-Bower & Burt, 1989) were delivered at a rate of 20 mm hr-1 for a 30 minute period on days 2, 9, 31 and 49 to examine the persistence of the P signal over time. Runoff samples were analyzed for TP and DRP using standard colorimetric techniques.
Results
The impacts of varying manure P composition were most evident in runoff TP concentrations on day 2 (Table 1). Reductions of P in runoff were not proportional to the reductions of P in manure. A maximum lowering of the P of manures (61%) resulted in a reduction of 39, 55 and 21 % in runoff TP in summer, autumn and spring, respectively.The percentage of TP as DRP in runoff was greatest at the highest P treatment in summer, 80%, compared to winter and spring at 60%.
Figure 2: Influence of soil moisture on runoff volumes
Conclusion
Reduced P diets show clear potential to reduce P concentrations in manure impacted runoff but effects were relatively short-lived. Strategies which lower interactions between applied manure and runoff could have a greater impact on P losses compared to varying dietary P.
Table 1: Mean TP and DRP concentrations in runoff 2 days following
manure application
Slurry P content
(%P)
Mean TP
(mg L-1)
Summer
Mean DRP
Autumn
Spring
1.3
1.0
0.9
0.5
Control
Probability
SEM
9.82a
8.32a
6.21b
5.94c
0.43d
0.004
1.34
7.83a
6.07a
6.69a
3.26b
0.20c
<0.001
0.732
13.77a
12.56a
7.52b
6.17c
1.42d
1.05
8.28a
7.86a
3.78b
2.18c
0.85d
0.572
6.78a
7.61a
7.05a
5.35b
0.93c
0.629
4.06a
4.88a
3.65b
2.11c
0.12d
0.378
References
Bowyer-Bower, T.A.S. & Burt, T.P., Rainfall simulators for investigating soil response to rainfall. Soil Technology 2, 1-16.
Ferris, C.P., McCann, M.E.E. & Patterson, D.C., Improving the efficiency of nitrogen and phosphorus use in livestock systems through dietary modification. Agricultural Research Institute of Northern Ireland, 78th Annual Report, 35-53
Sharon O’Rourke