Nitrogen and phosphorus mineralization potentials of soils receiving repeated annual cattle manure applications
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Manure application rates are generally calculated to balance nutrient inputs with crop requirements, based on a projected crop yield and estimates of nutrient release from recently applied manure during a growing season. Often, the contribution to plant nutrition of manure applied in the past is not considered explicitly. We obtained archived soil samples collected every 5 years during a 25-year period (1973–1998) from a long-term study in Lethbridge, Alberta, Canada to evaluate the effects of long-term manure applications on soil N and P mineralization potentials (Nmax and Pmax, respectively). Soils from experimental plots receiving 0, 30, 60, 90, 120 and 180 Mg manure (wet weight) ha–1 year–1 were incubated aerobically for 20 weeks under four different combinations of soil temperature (10°C and 20°C) and moisture [50% and 75% of field capacity (FC)] conditions. Nmax and Pmax were fit using a first-order rate equation. Nmax and Pmax were related linearly to the cumulative amount of N and P applied in manure, suggesting long-term manure applications increased the proportion of potentially mineralizable N and P in soils. Soil storage and handling in the laboratory (e.g., weekly rewetting during incubations) affected the slopes of the regression equations describing Nmax and Pmax. The slopes of regression lines relating Nmax and Pmax to cumulative manure applications were highest when soils were incubated at 20°C and 75% of FC. Adjusting manure application rates on agricultural land with a history of manure amendments, based on the increase in potentially mineralizable N and P from past manure applications, could help minimize nutrient export and environmental pollution from manure-amended soils.
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