MANURE PROPERTIES: EFFECT ON C AND N MINERALIZATION DURING LABORATORY INCUBATION OF MANURE AMENDED SOIL

Authors: Calderon, Francisco; Van Kessel, Jo Ann; McCarty, Gregory; Reeves Iii, James

Submitted to: USDA Symposium on Natural Resource Management to Offset Greenhouse Gas Emissions
Publication Type: Abstract Only
Publication Acceptance Date: 11/1/2002
Publication Date: 11/19/2002
Citation: Calderon, F.J., Van Kessel, J.S., Mccarty, G.W., Reeves Iii, J.B. 2002. Manure properties: effect on c and n mineralization during laboratory incubation of manure amended soil. USDA Symposium on Natural Resource Management to Offset Greenhouse Gas Emissions.

Interpretive Summary: The quality of a manure as an N fertilizer is difficult to predict, since manure properties may vary according to management variables. Laboratory incubations are performed in order to determine the N mineralization potential of manure-amended soils. However, these assays may take weeks to produce results, making them unsuitable for rapid manure management decisions. In addition, tight controls are necessary on sample pre-treatment, mixing, incubation time, sample disturbance, moisture, temperature, pre-leaching, extraction method, and aerobicity. Nitrogen losses through denitrification as well as N immobilization during laboratory incubations may be an additional source of error, affecting the correlation of incubation data with manure N mineralization in the field, as well as preventing good agreement manure N pools and mineralizable N. We hypothesize that denitrification and N immobilization may divert manure mineralizable N to N gas or microbial biomass, resulting in an inaccurate estimation of mineralizable N during laboratory incubations. Because of this, accounting for denitrified N will improve the correlation between manure N and mineralizable N. To achieve these objectives, we carried out six week laboratory incubations of manured soils. Soil mineral N, CO2 flux, and N2O flux after acetylene addition were measured weekly. Accomplishments include: a) Determined the effect of manure addition on soil C and N dynamics on a wide variety of manures, b) Determined if manure chemical or nutritional properties correlate to C and N pools, gas fluxes or denitrification during a laboratory incubation of manured soil, and c) Measured the amount of N lost through denitrification during an mineralizable N assay. Our results show that denitrification, N immobilization, and C mineralization increase in manured soils, and that the N cycling dynamics can not be accurately predicted by the manure chemical properties measured in this experiment. Denitrification accounted for a small fraction of the total manure N added and adding the total denitrified N to the N mineralized did not improve the predictive value of manure properties on mineralizable N.

Technical Abstract: A study was carried out to examine C and N mineralization in soils amended with dairy manure. Our objectives were to study the relationship between manure properties, mineralizable N, nutrient pools, gas fluxes, and denitrification during laboratory incubation. Different dairy manures (n=107) were added to soil at a rate of 0.1 mg manure-N per g soil. Manured and control soils were incubated and sampled weekly for soil mineral N, CO2 flux, and N2O flux for a six week period. The denitrifier enzyme activity (DEA) was measured at the end of the experiment. Weekly N2O and CO2 production increased in the manured soils during the first three weeks of incubation. There was a positive correlation between added manure C and cumulative CO2 production. The experimental and control soils increased in nitrate content throughout the six week period, but the increase was more marked in the manured soils. Ammonium concentration was initially high then declined in manured soils, resulting in no net accumulation of mineral N. Acetylene addition stimulated N2O production in the manured soils but not in the controls. No correlation existed between denitrifier enzyme activity (DEA) and N pools or gas fluxes in the manured soils. mg N2O-N kg-1 h-1 Negative net N mineralization accompanied by relatively low cumulative denitrification in the manured soils suggests that N immobilization occurred during the incubation. These results suggest that denitrifiers participate in the N dynamics during laboratory incubations, even when measures are taken to prevent anaerobicity.