Soil organic matter turnover rates increase to match increased inputs in grazed grasslands

Shane W. Stoner,Louis A. Schipper,Sebastian Doetterl,Carlos A. Sierra,Alison M. Hoyt,Marion Schrumpf,W. Troy Baisden,Susan Trumbore

doi:10.1007/s10533-021-00838-z

Abstract

Managed grasslands have the potential to store carbon (C) and partially mitigate climate change. However, it remains difficult to predict potential C storage under a given soil or management practice. To study C storage dynamics due to long-term (1952–2009) phosphorus (P) fertilizer and irrigation treatments in New Zealand grasslands, we measured radiocarbon (14C) in archived soil along with observed changes in C stocks to constrain a compartmental soil model. Productivity increases from P application and irrigation in these trials resulted in very similar C accumulation rates between 1959 and 2009. The ∆14C changes over the same time period were similar in plots that were both irrigated and fertilized, and only differed in a non-irrigated fertilized plot. Model results indicated that decomposition rates of fast cycling C (0.1 to 0.2 year−1) increased to nearly offset increases in inputs. With increasing P fertilization, decomposition rates also increased in the slow pool (0.005 to 0.008 year−1). Our findings show sustained, significant (i.e. greater than 4 per mille) increases in C stocks regardless of treatment or inputs. As the majority of fresh inputs remain in the soil for less than 10 years, these long term increases reflect dynamics of the slow pool. Additionally, frequent irrigation was associated with reduced stocks and increased decomposition of fresh plant material. Rates of C gain and decay highlight trade-offs between productivity, nutrient availability, and soil C sequestration as a climate change mitigation strategy.

Highlights

Grassland soils store approximately 340 Pg carbon (C) (Conant et al 2017), comprising about 23% of the global soil C stock to 1 m depth (Batjes 2016; FAO 2017) and covering roughly one third of global land surface (Rutledge et al 2017; McNally et al 2015)
Understanding the interactions between increasing inputs and decomposition is essential for informing management decisions to increase soil C stocks and extend the time this C is sequestered in soils (McSherry and Ritchie 2013; Conant et al 2017; Guo and Macdonald 2006)
Soil archiving allowed for a unique application of 14C to model long-term effects on C storage in changing, managed systems

Summary

Introduction

Grassland soils store approximately 340 Pg carbon (C) (Conant et al 2017), comprising about 23% of the global soil C stock to 1 m depth (Batjes 2016; FAO 2017) and covering roughly one third of global land surface (Rutledge et al 2017; McNally et al 2015). Roughly 70% of global grasslands are managed (Conant et al 2017), placing significant control of global soil C stocks into the hands of land managers. Management strategies for increasing soil C sequestration typically rely on increasing productivity (Conant et al 2001, 2017), a potential win–win approach. Understanding the interactions between increasing inputs and decomposition is essential for informing management decisions to increase soil C stocks and extend the time this C is sequestered in soils (McSherry and Ritchie 2013; Conant et al 2017; Guo and Macdonald 2006)

Methods

Results

Discussion

Conclusion