Constraining N cycling in the ecosystem model LandscapeDNDC with the stable isotope model SIMONE.

The isotopic composition (ic) of soil nitrogen (N) and, more recently, the intramolecular distribution of 15 N in the N2 O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N2 O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the stable isotope model for nutrient cycles (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here, we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of 15 N-N2 O and, to our knowledge for the first time, SP, to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N2 O emission, soil nitrate, and ammonium, as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly, the dynamics of 15 N-N2 O and SP of soil N2 O fluxes as simulated by SIMONE agreed well with measurements, though 15 N-N2 O was on average underestimated and SP overestimated (root-mean-square error [RMSE] of 8.4‰ and 7.3‰, respectively). Although 15 N-N2 O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10-59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high- frequency measurements of the N2 O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model-based strategies for mitigating N pollution.

Calpastatin prevents NF-κB-mediated hyperactivation of macrophages and attenuates colitis.

Calpain enzymes proteolytically modulate cellular function and have been implicated in inflammatory diseases. In this study, we found that calpain levels did not differ between intestinal tissues from inflammatory bowel disease (IBD) patients and healthy controls, but IBD tissues showed increased levels of the endogenous calpain inhibitor, calpastatin (CAST). To investigate the role of CAST in the immune system during IBD, mice were x-ray irradiated, reconstituted with either CAST-knockout (KO) or wild-type (WT) bone marrow, and subjected to dextran sulfate sodium-induced colitis. CAST-KO recipients with induced colitis exhibited more severe weight loss, bloody diarrhea, and anemia compared with WT controls. Histological evaluation of colons from KO recipients with colitis revealed increased inflammatory pathology. Macrophages purified from the colons of KO recipients had higher IL-6, TNF-α, and IFN-γ mRNA levels compared with WT controls. Mechanistic investigations using small interfering RNA and KO bone marrow to generate CAST-deficient macrophages showed that CAST deficiency during activation with bacterial pathogen associated molecular patterns, including heat-killed Enterococcus faecalis or CpG DNA, led to increased IκB cleavage, NF-κB nuclear localization, and IL-6 and TNF-α secretion. Thus, CAST plays a central role in regulating macrophage activation and limiting pathology during inflammatory disorders like IBD.