Plant, soil fauna and microbial responses to N-rich organic patches of contrasting temporal availability

A simple (L-lysine) 15N/13C dual-labelled organic patch was added to soil microcosm units with or without Lolium perenne L. plants either as a single addition of 5 ml of 200 mM L-lysine (‘patch’ treatment) or as a series of 1 ml aliquots of 200 mM L-lysine added at 7 day intervals over 28 days (‘pulse’ treatment) thus both treatments supplied the same amount of nitrogen (N). Controls were added as 1 ml H2O over 28 days (control pulse treatment) or as a single addition of 5 ml H2O (control patch treatment) to planted tubes. Decomposition of the added L-lysine was rapid as shown by amounts of 13C detected in the soil atmosphere and were greatest from the planted L-lysine pulse treatment indicating the presence of plant roots was influencing decomposition of the pulse. Plant uptake of N, as 15N, from the added L-lysine was also rapid and detected in the shoots by day 4. However, the mean rate at which 15N appeared in the shoots did not differ between patch or pulse treatments. No 13C levels greater than background were detected in the plant material. Root production and mortality in the patch or pulse addition zone was measured in situ using minirhizotron tubes. Cumulative root births in the L-lysine patch treatment were greater than controls in the latter part of the experiment while instantaneous root births were greater in the L-lysine pulse treatment compared to all others at 21 d. Root death rate was faster in the L-lysine treatments than in the controls. Root and shoot dry weights at final harvest (35 d) were greater in the pulse treatments. Shoot, but not root, dry weights were also significantly greater in the L-Lysine treatments compared to controls. Total root and shoot nitrogen contents were greater in the L-lysine treatments than controls. Total N capture by the L. perenne plants from the added L-lysine was 57% (patch) and 61% (pulse) of the N added and did not differ significantly between treatments. Protozoan biomass measured at harvest was greater in the L-lysine treatments (planted and unplanted) than the planted controls. The physiological profile of the microbial community did not vary because of patch or pulse treatments although significant differences between planted and unplanted L-lysine treatments and planted and H2O controls occurred. The response of roots and microorganisms in relation to N capture is discussed.