Effects of two soil reclamation techniques on the distribution of the organic N compounds in a 15N labelled burnt soil

Abstract

The evolution of the soil organic-N forms and their bio-availability was studied in a 15 N labelled and burnt soil (BLS) after two successive reclamation steps under greenhouse conditions: a 3-month growing period of Lolium, without (BLS-L) or with poultry manure addition (4 and 8 Mg ha − 1 : BLS + PM4-L and BLS + PM8-L), followed by a 12-month growing phase of pine seedlings (BLS-P, BLS + PM4-P and BLS + PM8-P). The results were compared with those obtained for the homologous labelled unburnt soil (LS, LS-L and LS-P) to evaluate the efficacy of these reclamation techniques in the mitigation of the drastic post-fire changes exhibited by the major biologically available N pool in terrestrial ecosystems: the soil organic N. The significant and steady decrease of the 15 N enrichment observed in the unburnt soil during the successive plant growth cycles (LS N LS-L N LS-P) contrasts with the lack of significant changes, in both the content of total organic 15 N and the atom % 15 N in excess, among the treatments with the burnt soil (BLS ≥ BLS-L ≥ BLS-P). These results showed that: a) in LS, N mineralization proceeds faster for the recently incorporated N ( 15 N enriched) than for the native N, supplying the growing vegetation with inorganic N more 15 N enriched than the bulk soil N; and b) in BLS, soil combustion has reduced the usually higher biological availability of the recently added N to levels similar to those of the endogenous N.The re-vegetation with Lolium and Pinus and the addition of poultry manure mitigated the high differences observed in the size of the amino acid and the organic derived NH 4 + -N pools due to the combustion process, which are usual between burnt and unburnt soils. Conversely, these burnt soil reclamation techniques (re-vegetation and poultry manure addition), even jointly used, were unable to reduce the huge differences observed between the burnt and the unburnt soils for the other N fractions considered (amides, amino sugars, hydrolysable unidentified-N, hydrolysable organic N and un-hydrolysable N) that accounted for more than 80% of the soil organic N. Consequently, it seems that without the introduction of N 2 -fixing microorganisms or plants in the burnt soils the recovery of the natural soil organic N composition will take place slowly.

Evaluation of legumes and poultry manure for the early protection of burnt soils .

“…Among organic amendments, Villar et al (1998) found better plant growth rates in a burnt soil amended with poultry manure as compared with cattle slurry or sewage sludge; moreover, Castro et al (2000) reported that poultry manure greatly enhanced gramineous plant growth even at low doses (1-4 Mg ha -1 ) enough to ensure its technical, economical and ecological applicability on burnt forest ecosystems. However, this technique is insufficient to recover the pre-fire status and distribution of soil organic N (Castro et al 2007).…”

Medium‐term effects of poultry manure on pine nitrogen uptake in a 15N‐labeled burnt soil§

“…This information is especially important considering that recently Castro et al (2007) found that revegetation and PM addition, even jointly used, were unable to reduce the huge differences observed between the burnt and the unburnt soils for N fractions that accounted for more than 80% of the soil organic N.…”

Availability of 15N from pioneer herbaceous plants to pine seedlings in reclaimed burnt soils

“…Among tree nutrients, N is very important because: (a) it is the major growth‐limiting mineral element for higher plants;17 and (b) its cycle is strongly altered by wildfires, which reduce dramatically the labile soil‐N reserves that are the major source of N for plants 18, 19. Therefore, from the N cycle point of view, evaluating accurately the medium‐term efficacy of the burnt soil restoration techniques is necessary; the usefulness of 15 N tracers for this task has been recently recognized 11, 12, 19–21…”