Modelling C and N dynamics with MAGIC model from annual to seasonal/monthly time step Filip Oulehle, Jack Cosby, Chris Evans, Jakub Hruška, Jiří Kopáček, Filip Moldan, Dick Wright

Introduction Effective immobilization of deposited nitrogen is a common feature of most acid sensitive catchments Saturation hypothesis suggests that N immobilization should decline Nitrate leaching may become important acidifying component Prediction of future N immobilization is probably the biggest uncertainty in acidification/eutrophication modelling

Modelling nitrogen with MAGIC MAGIC (Model for Acidification of Groundwater In Catchments) -Developed to predict the long-term effects of acidic deposition on surface water chemistry -Model simulates soil and surface water chemistry in response to changes in drivers such as deposition of S and N, land use practices, climate… - As sulphate concentrations have decreased, in response to the decreased S deposition, nitrate (NO 3 ) has become increasingly important. In acid soils much of the NO 3 leached from soil is accompanied by the acid cations H+ and inorganic aluminium (Ali) -In the early versions of MAGIC (version 1-5) retention of N was calculated empirically as a fraction of N deposited from input-output budgets -Later on fraction N retained was described as a function of the N richness of the ecosystem (soil C/N ratio in this case) – version 7

Soil C/N and N leaching - empirical evidence Some limitations - Soil C/N is vegetation specific - C/N ratio does not necessarily reflect the N-richness of the actively cycling component of the organic matter - C/N ratio does not appear to be useful in understanding relatively short-term changes in N dynamics - „hard“ to detect changes in soil C/N under field conditions Soil C/N seems to be a good predictor of N leaching on a spatial scale Lovett et al., Ecosystems (2002) 5: Oulehle et al. Ecosystems (2008) 11: 410–425

Modelling nitrogen with MAGIC In version 7 of MAGIC model – C/N soil ratio is the fundamental control on N leaching Limitation of this approach – example from Čertovo Lake MAGIC v5: N retention modelled as a first- order function of N deposition. MAGIC v7: N retention modelled as a function of N richness of the ecosystem Two shortcomings: 1) Over the short-term large changes in N leaching cannot be accounted for by changes in the C/N ratio since the C/N ration of soil organic matter changes only slowly. 2) The C/N ratio of bulk organic matter is in reality a consequence rather than the driver of the long-term retention and loss of N from the soil pool. Oulehle et al. Environmental Pollution (2012) 165: 158–166

Modelling nitrogen with MAGIC Alternative formulation of N retention in new version of MAGIC (MAGIC v7ext) is based directly on the microbial processes which determine the balance of N mineralization and immobilization. Conceptually developed by Jack Cosby - Inorganic N enters the model as deposition (wet and dry) - Time series of plant litter and N fixation (litter C and N) are external inputs to SOM. At each time step, decomposers process some of the C and N content of SOM (FC1 and FN1). A portion of this C and N turnover returns to the SOM as decomposer biomass (FC2 and FN2), while the remainder is lost from SOM as CO 2 and NH 4 (FC3 and FN3) or as DOC and DON (FC4 and FN4).

Modelling nitrogen with MAGIC Oulehle et al. Environmental Pollution (2012) 165: 158–166 Constant carbon turnover (FC1) The simulation can be further improved by including the negative effect of acidification on turnover of SOM during the period of peak S deposition – rationale well explained in Kopáček et al. (2013) Biogeochemistry 115: 33-51

Modelling nitrogen with MAGIC Summary: The new formulation of C and N processes in the soil gives a more satisfactorily simulation of the observed trends in NO3 in water compared to previous versions of the MAGIC model. The new formulation simulates both rapid (and amplified) ecosystem responses to changes in deposition of N, as well as the long-term changes in soil C/N resulting from chronic N deposition and accumulation in SOM. Limitations: Balanced C cycle, i.e. a constant soil C pool DOC and DON adjusted to fit the measured data

Modelling nitrogen with MAGIC Preliminary testing of MAGIC performance in monthly time step Soil organic matter decomposition and N uptake driven by changes in soil temperature – Q10 fce (calculated externaly) potential application in climate change scenario assessment Čertovo lake

Modelling nitrogen with MAGIC Seasonal MAGIC applied on four sites: Čertovo lake (CZ) – seasonal data available Gwy (Cymru) – Storgama (Norway) – Gårdsjön NITREX (Sweden) – DIN annual deposition

Modelling nitrogen with MAGIC The 4 sites are at various stages in N saturation

Modelling nitrogen with MAGIC Čertovo Lake inletGwyStorgamaGårdsjön Inputs mmol m -2 % of inputmmol m -2 % of inputmmol m -2 % of inputmmol m -2 % of input N depositon* Outputs N-NO 3 leaching N-NO 3 leaching observed DON leaching Denitrification Soil accumulation * In respect of Gårdsjön = deposition + fertilizer input N saturation Čertovo Lake >> Gwy > Storgama > Gårdsjön

Modelling nitrogen with MAGIC

Summary Despite reasonable model fit of cumulative N leaching across sites, only Čertovo calibration has shown satisfactory fit between modelled and observed NO 3 dynamic. This might be a result of uniform Q10 fce used across sites – need to try site specific Q10 for decomposition and N uptake Generally overestimation of N leaching during the winter months in Gwy - lack of proper winter? Decomposition and N uptake more tightly coupled at this heathland site? Presented examples pointed out that in Čertovo and Storgama catchments N dynamics behave quite similar, despite different level of N saturation (in other words Strogama might be fairly sensitive to N deposition). In respect of Gårdsjön, lack of seasonality in NO 3 leaching might be a consequence of artificial fertilizing. Are N addition experiments able to mimic altered N soil transformations caused by gradual enrichment through N deposition? Current MAGIC version is able to reproduce N seasonality as a result of coupled C and N dynamic Further development should focus on: soil C dynamic (C sequestration – more C soil pools?) feedback between soil acidity and C decomposition

Linkages between DOC availability and soil heterotrophic respiration Three mechanisms could lead to lower amount of bioavailable dissolved organic C (DOC) for the microbial community (Kopáček et al., 2013) (1)Increased abundance of N for plant uptake, causing lower C allocation to plant roots (2)Chemical suppression of DOC solubility by soil acidification (3) Enhanced mineralisation of DOC due to increased abundance of electron acceptors in the form of sulphate and nitrate - in anoxic soil micro-sites. Week 1Week 2Week 3Week 4 Soil analysis Leachate analysis Treatment addition CO 2 measurements Control H 2 SO 4 HClNaOHNaCl 800 ueq L -1

Linkages between DOC availability and soil heterotrophic respiration

Solution applications had immediate effect on DOC concentration in soil water.

Linkages between DOC availability and soil heterotrophic respiration Solution applications had immediate effect on DOC concentration in soil water and on soil respiration. In the end of the experiment, alkaline solution enhanced soil respiration by 20% compared to control, whereas acid treatment suppressed soil respiration by 15% compared to control. Neutral treatment has only short-term effect (suppression) on soil respiration.