Inorganic nitrogen additions to maize litter with different C/N ratios: Impacts on litter and soil organic matter decomposition

Background/Question/Methods

Most work addressing inorganic nitrogen (N) addition effects on carbon (C) cycling has been done in forested and other natural ecosystems.  However, maize-based agroecosystems have the potential to function differently than natural systems due to large inputs of monospecific litter with high C/N ratio and low lignin content.  Therefore, previous work on the relationship of inorganic nitrogen and carbon in natural systems where decomposers are primarily limited by carbon (energy) may not translate to agroecosystems where nutrient limitation may have more control over decomposers.  We hypothesized that the microbial community accesses N from SOM-N pool, in order to utilize litter-C for energy.  The addition of an easily usable N source such as an inorganic-N fertilizer will reduce the need for SOM-N and lead to faster litter decomposition and decreased SOM decomposition.  Further, while we know C/N ratio of litter can be used as a predictor of litter decomposition rates, little is known about how C/N ratio of litter could affect SOM-C decomposition.  We set up a 3 x 2 factorial experiment with soil and no litter addition, soil and maize leaf litter (C/N~40), and soil and maize stalk litter (C/N~120) with two levels of NH₄NO₃ fertilization (0 g/m² and 5 g/m²).  The soil used had been consistently cropped with wheat for 30+ years.  Thus, we could differentiate between litter (C₄ δ¹³C signature) and SOM-C (C₃ δ¹³C signature) decomposition.  We incubated these samples in the dark for 120 days at 25°C and measured CO₂ evolved and δ¹³C signature of evolved CO₂ over time. 

Results/Conclusions

We found that nitrogen addition increased litter decomposition of high C/N ratio stalk litter (f _{1, 12} =33.16 p=0.000), but had no effect on low C/N ratio leaf litter and an insignificant trend for reduced soil decomposition.  We found that the δ¹³C signature changed over time for each treatment (f _{4, 20}=486.40; p=0.000), but there was no impact of nitrogen addition on SOM-C decomposition (f _{3, 15}=1.45; p=0.268).  Our results show that while nitrogen can increase decomposition of high C/N ratio litter, it does not lead to decreased SOM-C decomposition.  Thus, there is a disconnect between litter and SOM decomposition, and while it may be easier to manipulate litter loss rates with N additions, that does not translate to any effects on SOM loss.