Nitrogen and phosphorus mineralization potentials of soils receiving repeated annual cattle manure applications

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 longterm study in Lethbridge, Alberta, Canada to evaluate the effects of long-term manure applications on soil N and P mineralization potentials (N-max and P-max, 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 degreesC and 20 degreesC) and moisture [50% and 75% of field capacity (FC)] conditions. N-max and P-max were fit using a first-order rate equation. N-max and P-max were related linearly to the cumulative amount of N and P applied in manure, suggesting longterm 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 N-max and P-max. The slopes of regression lines relating N-max and P-max to cumulative manure applications were highest when soils were incubated at 20 degreesC 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.