The European carbon balance. Part 2: croplands

Abstract

We estimated the long-term carbon balance [net biome production (NBP)] of European (EU-25) croplands and its component fluxes, over the last two decades. Net primary production (NPP) estimates, from different data sources ranged between 490 and 846 gC m−2 yr−1, and mostly reflect uncertainties in allocation, and in cropland area when using yield statistics. Inventories of soil C change over arable lands may be the most reliable source of information on NBP, but inventories lack full and harmonized coverage of EU-25. From a compilation of inventories we infer a mean loss of soil C amounting to 17 g m−2 yr−1. In addition, three process-based models, driven by historical climate and evolving agricultural technology, estimate a small sink of 15 g C m−2 yr−1 or a small source of 7.6 g C m−2 yr−1. Neither the soil C inventory data, nor the process model results support the previous European-scale NBP estimate by Janssens and colleagues of a large soil C loss of 90 ± 50 gC m−2 yr−1. Discrepancy between measured and modeled NBP is caused by erosion which is not inventoried, and the burning of harvest residues which is not modeled. When correcting the inventory NBP for the erosion flux, and the modeled NBP for agricultural fire losses, the discrepancy is reduced, and cropland NBP ranges between −8.3 ± 13 and −13 ± 33 g C m−2 yr−1 from the mean of the models and inventories, respectively. The mean nitrous oxide (N₂O) flux estimates ranges between 32 and 37 g C Eq m−2 yr−1, which nearly doubles the CO₂ losses. European croplands act as small CH₄ sink of 3.3 g C Eq m−2 yr−1. Considering ecosystem CO₂, N₂O and CH₄ fluxes provides for the net greenhouse gas balance a net source of 42–47 g C Eq m−2 yr−1. Intensifying agriculture in Eastern Europe to the same level Western Europe amounts is expected to result in a near doubling of the N₂O emissions in Eastern Europe. N₂O emissions will then become the main source of concern for the impact of European agriculture on climate

Agricultural management explains historic changes in regional soil carbon stocks

“…LoA1 indicate well to moderately drained soils and LoA2 the poorly drained soils in the loam region. account when process-based models are run at a regional scale (21). The episodic nature of such interventions explains part of the differences in the results of repeated SOC inventories and also the differences between such inventories and regional SOC models.…”

Biomass turnover time in terrestrial ecosystems halved by land use

“…Cropland NPPact was taken from previous work, which extrapolated NPP from primary crop harvest, applying region-and plant-specific factors such as dry matter content and harvest indices, as well as factors for pre-harvest losses 49,50 . For non-cropland areas, NPPact was assessed by calculating differences to NPPpot due to management 15,20,21 .…”

Potential carbon sequestration of European arable soils estimated by modelling a comprehensive set of management practices

“…At European level, several long-term soil monitoring networks have reported soil organic carbon (SOC) decreases (Bellamy et al, 2005;Goidts & van Wesemael, 2007;Capriel, 2013;Heikkinen et al, 2013), even if contrasting trends are evident in the past decades (Chapman et al, 2013;Reynolds et al, 2013). Weak SOC trends are likely to mask the intrinsic variability in SOC detection, especially as soils under cropland show currently a carbon balance close to a steady-state condition in Europe (Ciais et al, 2010).…”