Chemical characterization of manure in relation to manure quality as a contribution to a reduced nitrogen emission to the environment

Keywords:manure composition, ammonia volatilization, free ions, Donnan Membrane Technique, manure additives, dietary changes, nitrogen dynamics,grasslandsoils.More insight in manure composition, ammonia (NH 3 ) volatilization, and into relationships between soil organic matter (SOM) content and nitrogen (N) flows in grasslands is needed to improve the nutrient use efficiency of manure as fertilizer for plants and concomitantly to reduce the environmental impact of agricultural activities.Additives are often added to improve manure quality ( e.g. reduce NH 3 volatilization). Laboratory studies showed that additives used in this study did in general not affect manure characteristics or NH 3 volatilization, when manure was incubated at three temperatures and either regular mixed or not mixed. Only when Agri-mest ® and Effective Micro-organisms ® were both added and the manure was incubated at 4 °C without mixing, NH 3 volatilization was reduced by 34%.To study manure composition in more detail, a DMT-manure cell was developed, which either measures 'free' dissolved cation or anion concentrations in manure. Dilution studies showed that total nutrient concentrations and monovalent 'free' dissolved cation concentrations decreased proportionately with increasing dilution, whereas 'free' dissolved divalent cation concentrations were buffered upon dilution. The buffering of divalent cation concentrations is probably the result of the release of these ions from organic matter, or it is the result of the dissolution of certain phosphate minerals ( e.g. struvite and whitlockite). Only a small part of the total phosphorus (P) content (<1%) and the total sulphur content (<13%) was present in a 'free' dissolved ionic form.A feeding trial with non-lactating cows showed that the total N and P content and the total ammoniacal nitrogen (TAN= NH 3,aq + NH 4+ ) content of manure increased when the dietary protein or energy content was raised, whereas the 'free'-to-total ratios of Ca and Mg in the manures were not affected by dietary changes. Ammoniavolatilization from manure increased with an increase in dietary protein content and decreased with an increase in dietary energy content, although the TAN content of manures produced from diets with a higher dietary energy content werehigher.A field trial, in combination with incubation studies, was carried out in the VEL-VANLA area to study how N dynamics was affected by SOM content. Incubation studies showed that potential N mineralization rates (measured at 20 °C and 60% water holding capacity) of eighteen grassland soils were related to the initial dissolved organic nitrogen content and to the dissolved organic carbon content of the soil, as measured in 0.01M CaCl 2 extracts. During the growing season, two peaks in total soluble N content were observed, which could not be related to the time of manure application, but were both preceded by a period of draught. Maximum total soluble N and nitrate content were higher in soils with a lower SOM content. This is probably caused by faster water depletion of soils with a lower SOM content during drought periods, which will hamper N uptake by plants.Concluding, adjusting diets seems to be a more promising way to reduce NH 3 volatilization than the use of manure additives, especially since adjusting the diet will reduce NH 3 volatilization at any time of manure handling (housing, storage, and during manure application), which is not always the case for other NH 3 volatilization reduction techniques. A proper estimation of N mineralization during the growing season can help to improve the nutrient gift (via manure) in order for plants to grow with limited risks to the environment. Dissolved organic N might be a promising parameter to estimate N mineralization.