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

Manure application rates are generally calculated to balance nutrient input s with crop requirements, based on a projected crop yield and estimates of nutrient release from recently applied manure during a growing season. Ofte n, 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 Lethbri dge, Alberta, Canada to evaluate the effects of long-term manure applicatio ns 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 ma nure (wet weight) ha(-1) year(-1) were incubated aerobically for 20 weeks u nder four different combinations of soil temperature (10 degreesC and 20 de greesC) 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, su ggesting longterm manure applications increased the proportion of potential ly mineralizable N and P in soils. Soil storage and handling in the laborat ory (e.g., weekly rewetting during incubations) affected the slopes of the regression equations describing N-max and P-max. The slopes of regression l ines relating N-max and P-max to cumulative manure applications were highes t 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 manur e applications, could help minimize nutrient export and environmental pollu tion from manure-amended soils.