Abstract
Highlights
•15N-labelled urine was traced into plant, microbial and leaching pools in a grassland mesocosm.
•Leaching losses were reduced, and plant uptake increased during periods of low rainfall.
•Microbial uptake of urinary-N occurred via mineralisation-immobilisation and intact pathways.
•Microbial assimilation was dominated by bacteria over fungi.
Abstract
Urine patches in grazed systems are hotspots for nitrogen (N) cycling and losses to the wider environment. Retention and subsequent recycling of urinary-N is key to minimise losses and increase ecosystem nitrogen use efficiency. Biosynthesis into the microbial organic N pool is an important N pathway but this has not been directly quantified in a urine patch. Herein, we present the results of a time course experiment using soil mesocosms sown with perennial ryegrass (Lolium perenne L.) and treated with 15N-labelled sheep urine to determine partitioning of the applied N between plant, soil biomass pools and leaching losses following simulated rainfall events. 15N-tracing used bulk and compound-specific 15N-stable isotope probing (SIP) to determine the fate of urinary N. Initial high leaching losses (233 kg N ha−1) were comprised of native soil N, ammonium and nitrate derived from urine by urea hydrolysis and nitrification, respectively. Leaching subsequently decreased whilst uptake into plant biomass and microbial biosynthesis increased during periods of low rainfall. Uptake into above and belowground plant biomass was the largest fate of urinary-15N after 94 d (42%), although assimilation into microbial biomass dominated for ca. 1 month after urine deposition (34%). Compound-specific 15N–SIP of amino acids and amino sugars revealed immobilisation of urinary-N following mineralisation was the dominant pathway for biosynthesis, with incorporation into bacterial organic N pools more rapid than into the fungal biomass. There was also intact utilisation of glycine derived from urine. This study provides clear evidence that direct assimilation of urine-derived N into microbial organic N pools is an important process for retaining N in a urine patch, which will subsequently support plant N supply during microbial turnover.
•15N-labelled urine was traced into plant, microbial and leaching pools in a grassland mesocosm.
•Leaching losses were reduced, and plant uptake increased during periods of low rainfall.
•Microbial uptake of urinary-N occurred via mineralisation-immobilisation and intact pathways.
•Microbial assimilation was dominated by bacteria over fungi.
Abstract
Urine patches in grazed systems are hotspots for nitrogen (N) cycling and losses to the wider environment. Retention and subsequent recycling of urinary-N is key to minimise losses and increase ecosystem nitrogen use efficiency. Biosynthesis into the microbial organic N pool is an important N pathway but this has not been directly quantified in a urine patch. Herein, we present the results of a time course experiment using soil mesocosms sown with perennial ryegrass (Lolium perenne L.) and treated with 15N-labelled sheep urine to determine partitioning of the applied N between plant, soil biomass pools and leaching losses following simulated rainfall events. 15N-tracing used bulk and compound-specific 15N-stable isotope probing (SIP) to determine the fate of urinary N. Initial high leaching losses (233 kg N ha−1) were comprised of native soil N, ammonium and nitrate derived from urine by urea hydrolysis and nitrification, respectively. Leaching subsequently decreased whilst uptake into plant biomass and microbial biosynthesis increased during periods of low rainfall. Uptake into above and belowground plant biomass was the largest fate of urinary-15N after 94 d (42%), although assimilation into microbial biomass dominated for ca. 1 month after urine deposition (34%). Compound-specific 15N–SIP of amino acids and amino sugars revealed immobilisation of urinary-N following mineralisation was the dominant pathway for biosynthesis, with incorporation into bacterial organic N pools more rapid than into the fungal biomass. There was also intact utilisation of glycine derived from urine. This study provides clear evidence that direct assimilation of urine-derived N into microbial organic N pools is an important process for retaining N in a urine patch, which will subsequently support plant N supply during microbial turnover.
| Original language | English |
|---|---|
| Article number | 109011 |
| Number of pages | 11 |
| Journal | Soil Biology and Biochemistry |
| Volume | 180 |
| Early online date | 16 Mar 2023 |
| DOIs | |
| Publication status | Published - 1 May 2023 |
Bibliographical note
Funding Information:MKR, RPE and DLJ received funding from UK Natural Environment Research Council as part of the NERC large grant DOMAINE (Dissolved Organic Matter in Freshwater Ecosystems; NE/K010905/1 , and NE/K01093X/1 ). KAM, DRC and DLJ received funding from NERC under the grant award NE/M015351/1 (Uplands-N 2 O). Stable nitrogen isotope analysis of feeding experiment samples was undertaken at the Lancaster node of the LSMSF facility, funded by NERC . The authors wish to thank the NERC for partial funding of the National Environmental Isotope Facility (NEIF; contract no. NE/V003917/1 ).
Funding Information:
Llinos Hughes and Mark Hughes are thanked for support at Henfaes Research Station. Jonathon Pemberton, Gordon Inglis, Jon Holmberg and Emily Cooledge are thanked for assistance with the collection of 15 N-labelled urine. Alison Kuhl and Iain Kendall are thanked for assistance with compound-specific amino acid analyses. Fotis Sgouridis, Timothy Knowles and Paul Monaghan are thanked for assistance with bulk 15 N and TN analyses. The authors wish to thank the HEFCE SRIF and the University of Bristol for funding the GC-IRMS capabilities.
Publisher Copyright:
© 2023 The Authors