Variable effects of nitrogen additions on the stability and turnover of soil carbon

Abstract

Soils contain the largest near-surface reservoir of terrestrial carbon and so knowledge of the factors controlling soil carbon storage and turnover is essential for understanding the changing global carbon cycle. The influence of climate on decomposition of soil carbon has been well documented, but there remains considerable uncertainty in the potential response of soil carbon dynamics to the rapid global increase in reactive nitrogen (coming largely from agricultural fertilizers and fossil fuel combustion). Here, using 14C, 13C and compound-specific analyses of soil carbon from long-term nitrogen fertilization plots, we show that nitrogen additions significantly accelerate decomposition of light soil carbon fractions (with decadal turnover times) while further stabilizing soil carbon compounds in heavier, mineral-associated fractions (with multidecadal to century lifetimes). Despite these changes in the dynamics of different soil pools, we observed no significant changes in bulk soil carbon, highlighting a limitation inherent to the still widely used single-pool approach to investigating soil carbon responses to changing environmental conditions. It remains to be seen if the effects observed here-caused by relatively high, short-term fertilizer additions-are similar to those arising from lower, long-term additions of nitrogen to natural ecosystems from atmospheric deposition, but our results suggest nonetheless that current models of terrestrial carbon cycling do not contain the mechanisms needed to capture the complex relationship between nitrogen availability and soil carbon storage.

How relevant is recalcitrance for the stabilization of organic matter in soils?

“…However, several recent studies have shown, that a selective preservation of lignin appears to be only relevant during the early stages of litter decomposition and that later-on lignin degradation occurs at the same or even higher rate as the overall litter decomposition (Gleixner et al, 1999;Kerem et al, 1999;Jensen et al, 2005;Prescott, 2005;Kalbitz et al, 2006;Sollins et al, 2006). Recent studies using 13 C-CPMAS-NMR and pyrolysis techniques have confirmed that lignin is altered relatively quickly and does not appear to be stabilized in the long-term in any soil fraction (Baldock and Nelson, 2000;Gleixner et al, 2002;Kiem and Kögel-Knabner, 2003).…”

Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

“…Our previous field investigations have shown that fungal biomass and fungi to bacteria ratios (Wang et al, 2015) and decomposition of lower-order roots and needles (Kou et al, 2015a(Kou et al, , 2015b) exhibited a decreasing trend at high NH 4 þ additions. Moreover, long-term elevated N input has been found to enhance chemical stabilization of organic matter into recalcitrant compounds that are resistant to microbial decay (Neff et al, 2002;Swanston et al, 2004), thereby potentially impairing fungal metabolism (Maaroufi et al, 2015). With increasing additions of NH 4 þ , microbial NH 4 þ cycling shifted from a state of decoupling (gross NH 4 þ immobilization rates were incomparable to gross N mineralization rates) to coupling (gross NH 4 þ immobilization rates were comparable to gross N mineralization rates).…”

Estoques de carbono e nitrogênio em frações da matéria orgânica de solos cultivados com eucalipto nos sistemas convencional e fertirrigado

“…Porém, a fertilização, principalmente com adição de N, pode auxiliar diretamente na estabilização de formas orgânicas do C no solo, aumentando, também, os estoques de C e N em frações mais humificadas da MOS, conforme verificado no sistema fertirrigado. NEFF et al (2002) demonstraram que a fertilização com N acelerou a decomposição das frações leves da matéria orgânica com tempos de ciclagem em torno de décadas, mas auxiliou na estabilização das frações de C das substâncias húmicas associados à fração mineral e que possuem tempo de ciclagem de décadas (STEVENSON, 1994). Tal hipótese é corroborada pelo incremento nos estoques de N nas substâncias húmicas, em especial nos ácidos húmicos e huminas (Figura 2).…”

“…recalcitrantes, abundantes em resíduos vegetais de eucalipto, favorecendo seu acúmulo nas substâncias húmicas, pela sua menor suscetibilidade à ação de enzimas lignolíticas (NEFF et al, 2002).…”

“…A adição desses elementos em solos altamente intemperizados e inférteis tem elevado a produtividade do eucalipto (NEFF et al, 2002;LEGGETT & KELTING, 2006;STAPE et al, 2006). Além disso, trabalhos têm demonstrado que os estoques de C e N nos compartimentos do solo podem aumentar (LEGGETT & KELTING, 2006;LI et al, 2006) ou diminuir (NEFF et al, 2002) em razão da suplementação de nutrientes. A irrigação também aumenta a produtividade das plantas (STAPE et al, 2006) e pode contribuir para aumentar a MOS (RASSE et al, 2005).…”