Global cropland monthly gross primary production in the year 2000

T Chen,N Gobron,A J Dolman,G R Van Der Werf,E J Moors

doi:10.5194/bg-11-3871-2014

Abstract

Abstract. Croplands cover about 12% of the ice-free terrestrial land surface. Compared with natural ecosystems, croplands have distinct characteristics due to anthropogenic influences. Their global gross primary production (GPP) is not well constrained and estimates vary between 8.2 and 14.2 Pg C yr−1. We quantified global cropland GPP using a light use efficiency (LUE) model, employing satellite observations and survey data of crop types and distribution. A novel step in our analysis was to assign a maximum light use efficiency estimate (&amp;varepsilon;*GPP) to each of the 26 different crop types, instead of taking a uniform value as done in the past. These &amp;varepsilon;*GPP values were calculated based on flux tower CO2 exchange measurements and a literature survey of field studies, and ranged from 1.20 to 2.96 g C MJ−1. Global cropland GPP was estimated to be 11.05 Pg C yr−1 in the year 2000. Maize contributed most to this (1.55 Pg C yr−1), and the continent of Asia contributed most with 38.9% of global cropland GPP. In the continental United States, annual cropland GPP (1.28 Pg C yr−1) was close to values reported previously (1.24 Pg C yr−1) constrained by harvest records, but our estimates of &amp;varepsilon;*GPP values were considerably higher. Our results are sensitive to satellite information and survey data on crop type and extent, but provide a consistent and data-driven approach to generate a look-up table of &amp;varepsilon;*GPP for the 26 crop types for potential use in other vegetation models.

Highlights

The terrestrial biosphere assimilates an estimated 120– 150 Pg C yr−1 (Beer et al, 2010; Welp et al, 2011) as gross primary production (GPP)
Because both the εG∗ PP values based on biomass as well as the FLUXNET-based values are relatively high, the values used in our model are higher than those used in other models (Zhao and Running, 2010; Lobell et al, 2002; Field et al, 1995; Potter et al, 1993)
Even if we double the 0.5 g C MJ−1 number to account for the GPP / net primary production (NPP) ratio of about 2, the value is still much lower than the εG∗ PP values we found here

Summary

Introduction

The terrestrial biosphere assimilates an estimated 120– 150 Pg C yr−1 (Beer et al, 2010; Welp et al, 2011) as gross primary production (GPP). Half of the GPP is used for plant maintenance processes and is generally referred to as autotrophic respiration (Ra). The remainder is available for plant growth as net primary production (NPP), which is subsequently consumed mostly by heterotrophs (Rh) and fire. Biochemical processes of photosynthesis at cell or leaf level are relatively well known, but accurate estimates of GPP at larger scales (regional or global) are still uncertain. Current global GPP estimates still mainly rely on model results. Beer et al (2010) reported global cropland GPP of 14.8 Pg C yr−1using flux tower measurements based on eddy covariance methods and several diagnostic models. Saugier et al (2001) estimated this number to be 8.2 Pg C yr−1

Objectives

Methods

Results

Discussion

Conclusion