Land use controls the balance between autotrophic and heterotrophic respiration in lowland streams

Agricultural land use fundamentally alters stream ecosystems, but previous studies show inconsistent effects on ecosystem respiration (ER), with little attention to autotrophic versus heterotrophic components. This study aimed to analyze how agricultural intensity affects gross primary production (GPP) and the balance between autotrophic respiration (AR) and heterotrophic respiration (HR). We used data from measurements of GPP and ecosystem respiration (ER) using open-water metabolism measurements in 25 lowland streams spanning an agricultural gradient (0-84.5% coverage) in the Nordic region. We separated ER into AR and HR using the Hall and Beaulieu partitioning framework. Contrary to many studies, GPP did not increase with agricultural land cover or nutrients but instead correlated with stream depth and plant coverage. However, the heterotrophic fraction of respiration (HR/ER) increased significantly with agricultural intensity, with agriculture explaining 56% of the variation (HR/ER increased 0.004 per 1% agriculture increase). Heterotrophic respiration comprised up to 94% of total respiration in the most agricultural catchments (median: 84%). This shift occurred despite wide variation in stream characteristics, indicating that catchment land use, not in-stream conditions, drives the metabolic regime. We propose that tile drainage and manure applications increase the quantity and lability of terrestrial organic carbon entering streams, fueling heterotrophic metabolism. This shift has critical implications for CO 2 emissions, as heterotrophic respiration represents net conversion of terrestrial carbon to atmospheric CO 2 . Our findings suggest HR/ER could serve as a functional indicator of agricultural impacts and that agricultural carbon accounting should include stream-mediated CO 2 losses.