Origin of carbon in agricultural soil profiles deduced from depth gradients of C:N ratios, carbon fractions, \u03b413C and \u03b415N values

Florian Schneider,Wulf Amelung,Axel Don

doi:10.1007/s11104-020-04769-w

Florian Schneider, Wulf Amelung + Show 1 more

Open Access

https://doi.org/10.1007/s11104-020-04769-w

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Abstract

AimsAgricultural soils in Germany store 2.54 Pg of organic carbon (C). However, information about how and when this C entered the soils is limited. This study illustrates how depth profiles of organic matter can shed light on different entry paths of organic C.MethodsMachine learning was used to explain total organic C (TOC), C:N, particulate organic C (POC), δ13C and δ15N values down to 100 cm depth based on pedology, geology, climate and management-related variables from the German Agricultural Soil Inventory. We estimated TOC turnover rates based on the relationship between the proportion of maize (only C4 plant) in crop rotations and soil δ13C values.ResultsIn the upper 30 cm of cropland, fresh photosynthates added on average 0.2 to 0.8 Mg C ha− 1 year− 1. Organic fertiliser was another source of topsoil C, especially in grassland. Sandy sites in north-west Germany contained historic C from past heathland and peatland. One third of German agricultural land was found to be on colluvial and alluvial deposits, in which allochthonous C from upstream and upslope areas evidently increased the TOC content of subsoils. In and below hardpans, TOC content and C:N and POC:TOC ratios were low, indicating restricted root-derived C input.ConclusionsOur data indicate that ongoing management in German agricultural soils mainly affects topsoil C, while C storage in subsoils reveals significant legacies from allochthonous, buried or translocated C inputs. Specific attention should be focused on the sustainable loosening of hardpans that could result in a slow, but significant increase in subsoil C stocks.

Highlights

Soils contain about 1,500 Pg of carbon bound within soil organic matter in the upper 100 cm (Batjes 1996)
Total organic C (TOC), total inorganic C (TIC) and total nitrogen were measured by dry combustion using an elemental analyser (LECO TRUMAC, St Joseph, MI, USA)
As processes governing C stabilisation in organic soil differ from those in mineral soils, and as root-derived C inputs contribute only little to C stocks of organic soil, this study focused solely on mineral soils

Summary

Introduction

Soils contain about 1,500 Pg of carbon bound within soil organic matter in the upper 100 cm (Batjes 1996). This is about twice the amount of carbon (C) that currently resides in the Earth’s atmosphere as carbon dioxide (IPCC 2013). Primary organic C inputs may originate from: (i) aboveground biomass that is not harvested, such as stubble, mulch and green manure, (ii) organic fertilisers including animal excreta, compost and biogas digestates, and (iii) root litter and rhizodeposits. The relative contribution of aboveground biomass, organic fertiliser and roots to total C inputs varies considerably between different land uses and management regimes. For instance, receives more C input via roots and rhizodeposits than cropland (Pausch and Kuzyakov 2018), while cropland receives a larger C input from aboveground in the form of harvest residues and stubble

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