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TECHNICAL REPORTS

Waste Management

Co-digestion of Animal Slurry Can Increase Short-Term Nitrogen Recovery by Crops

Animal Production Div. of the Animal Sciences Group of Wageningen UR, P.O. Box 65, NL-8200 AB Lelystad, The Netherlands

^* Corresponding author (Herman.deBoer{at}wur.nl).

Received for publication November 9, 2007. Co-digestion changes slurry characteristics and is supposed to increase short-term nitrogen (N) uptake by crops after application. A higher N uptake from slurry reduces the need for additional mineral N fertilizer. If farmers apply co-digested slurry (CS), a higher N recovery has to be taken into account to prevent losses to the environment. Since data on the effects of co-digestion on N recovery by crops are scarce, a pot experiment was performed. The apparent N recovery (ANR) of five different co-digested pig slurries was compared with their raw source slurries (RS) during 105 d after a single fertilization of ryegrass (Lolium perenne L.), grown under controlled conditions. Slurry was mixed with sandy soil and grass was cut every 35 d. The results show that co-digestion increased (p < 0.05) the ANR at first cut on average from 39 to 50%, at second cut from 7 to 9% (p < 0.05), and had no effect on ANR at third cut (3%). The ANR increase at first cut was likely due to an increase of the NH₄–N/total N ratio along with a decrease of the organic C/total N ratio of slurry during co-digestion. Field application may under certain circumstances decrease N fertilizer value of CS, due to a higher NH₃ emission compared to RS. A potential ANR increase may then be reduced, absent, or even become a decrease. Under comparable NH₃ emissions, however, CS can in the short term be more valuable as an N fertilizer than RS, and fertilizer savings can likely be realized.

Abbreviations: ANR, apparent nitrogen recovery • CS, co-digested slurry • LSD, least significant difference • REML, residual maximum likelihood • RS, undigested raw slurry • SED, standard error of deviation.