Modelling of grassland fluxes in Europe

Two independently developed simulation models – the grassland-specific PaSim and the biome-generic Biome-BGC MuSo (BBGC MuSo) – linking climate, soil, vegetation and management to ecosystem biogeochemical cycles were compared in a simulation of carbon (C) and water fluxes. The results were assessed against eddy-covariance flux data from five observational grassland sites representing a range of conditions in Europe: Grillenburg in Germany, Laqueuille in France with both extensive and intensive management, Monte Bondone in Italy and Oensingen in Switzerland. Model comparison (after calibration) gave substantial agreement, the performances being marginal to acceptable for weekly-aggregated gross primary production and ecosystem respiration (R2 ~0.66-0.91), weekly evapotranspiration (R2 ~0.78-0.94), soil water content in the topsoil (R2 ~0.1-0.7) and soil temperature (R2 ~0.88-0.96). The bias was limited to the range -13 to 9 g C m-2 week-1 for C fluxes (-11 to 8 g C m-2 week-1 in case of BBGC MuSo, and -13 to 9 g C m-2 week-1 in case of PaSim) and -4 to 6 mm week-1 for water fluxes (with BBGC MuSo providing somewhat higher estimates than PaSim), but some higher relative root mean square errors indicate low accuracy for prediction, especially for net ecosystem exchange.

 Temporal course of simulated (two models, without and with calibration) and observed weekly averaged ST (°C), SWC (m³ m^-3), and aggregated ET (mm week^-1), GPP, RECO and NEE (g C m^-2 week^-1) data at Laqueuille intensive (France) for 2004-2010 (source: supplementary matterial).

The sensitivity of simulated outputs to changes in atmospheric carbon dioxide concentration ([CO2]), temperature and precipitation indicate, with certain agreement between the two models, that C outcomes are dominated by [CO2] and temperature gradients, and are less due to precipitation. ET rates decrease with increasing [CO2] in PaSim (consistent with experimental knowledge), while lack of appropriate stomatal response could be a limit in BBGC MuSo responsiveness. Results of the study indicate that some of the errors might be related to the improper representation of soil water content and soil temperature. Improvement is needed in the model representations of soil processes (especially soil water balance) that strongly influence the biogeochemical cycles of managed and unmanaged grasslands.

Read more details in:

Sándor, R., Barcza, Z., Hidy, D., Lellei-Kovács, E., Ma, S., Bellocchi, G. (2016): Modelling of grassland fluxes in Europe: Evaluation of two biogeochemical models. Agriculture, Ecosystem and Environment, 215, 1-19.

Spatial variability of hydrophysical properties of fallow sandy soils

Spatial heterogeneity of soil hydrophysical properties was estimated in 2 fallow sandy soils at Csólyospálos and Őrbottyán, Hungary. Significant differences in small particle (= silt + clay) content (5.0% vs. 13.7%), organic matter content (1.62% vs. 0.91%), and CaCO3 content (3.1% vs. 5.1%) measured at Csólyospálos and Őrbottyán, respectively, resulted in a higher persistence of water repellency in the Csólyospálos soil. It also resulted in a significantly higher water sorptivity and hydraulic conductivity of the Őrbottyán soil. 

Scatterplot correlation matrix of hydrophysical properties at CSP site, where k: field saturated hydraulic conductivity; Sw: water sorptivity; Swh: water sorptivity of hydrophobic soil state; Sww: water sorptivity of nearly wettable soil state; WRCT: water repellency cessation time; WDPT: water drop penetration time.

The spatial heterogeneity of soil hydrophysical properties was significant reaching 3 orders of magnitude differences due to the variances of soil properties. The water repellency cessation time was inversely related to the hydraulic conductivity and water sorptivity at Csólyospálos site.

More details in:
Sándor, R., Lichner, L., Filep, T., Balog K., Lehoczky É., Fodor, N. (2015): Spatial variability of hydrophisical properties of fallow sandy soils. Biologia, 70/11, 1

Spatial patterns of wetting characteristics

In grasslands where organic and inorganic resources are alternating at scales of individual plants, the transient character is given to certain wetting properties of soil, which then become highly variable both in space and in time. The objective of presented study was to study wetting pattern within two soil horizons at 5-cm and 10-cm depths respectively and to examine how the wetting patterns relate to hydraulic conductivity determined by Minidisc infiltrometer at suction –2 cm, K(–2 cm). 

Field preparation and hydraulic conductivity measurements with Minidisc infiltrometer at Sekule (Slovakia).

This characteristics is implicitly independent on antecedent soil water content (SWC) since it relates to steady infiltration phase but can be influenced by present soil water repellency (SWR). Field measurements were per-formed on July 27–28, 2010 on the grassland experimental site located near the village Sekule in Southwest Slovakia. The water drop penetration time (WDPT), SWC and tension Minidisc infiltration measurements were carried out on the 0.64 m2 plot in a regular 8 x 8 grid. The results showed that SWR and SWC influence each other and cause correlation between spatial patterns of studied soil wetting characteristics and between characteristics measured at the two soil depths. Further, it was found out, that calculation of K(–2 cm) according to Zhang may cause apparent correlation of K(–2 cm) with antecedent SWC, which is the artificial effect of sorptivity parameter in the equation on steady stage of infiltration process. This pseudocorrelation has disappeared after adopting of Minasny and McBratney (2000) approaches by calcu-lation of K(–2 cm).

More details in:
Orfánus, T., Stojkovová, D., Rajkai, K., Czachor, H., Sándor, R. (2016): Spatial patterns of wetting characteristics in grassland sandy soil. Journal of Hydrology and Hydromechanics, 64.

Relations between maize, weed, soil water content and fertilization treatments

Especially during early developmental stages, competition with weeds can reduce crop growth and have a serious effect on productivity. Here, the effects of interactions between soil water content (SWC), nutrient availability, and competition from weeds on early stage crop growth were investigated, to better understand this problem. Field experiments were conducted in 2013 and 2014 using long-term study plots on loam soil in Hungary. Plots of maize (Zea mays L.) and a weed-maize combination were exposed to five fertilization treatments. SWC was observed along the 0–80 cm depth soil profile and harvested aboveground biomass (HAB) was measured. 

        Significant differences were found between SWC in maize and maize-weed plots. In all treatments, measured SWC was most variable in soil depths of up to 50 cm, and at the 8–10 leaves (BBCH19) growth stage of the crop.

 

Two-year volumetric SWC (%) changes at the 10–20 cm soil depth layer for the 2−4 leaf (BBCH 13) (a, b and c) and 8−10 leaf (BBCH 19) (d, e and f) maize growth stages at Nagyhӧrcsӧk (Hungary) under maize and maize-weed vegetation and 5 fertilization treatments (Control, NK, NP, PK, NPK). a., d. Boxes are delimiting the 25th and 75th percentiles with the median inside. Whiskers are 10th and 90th percentiles. SWC under maize appears with yellow shaded background whilst the maize-weed combination with green. Hollow circles indicate outliers. b., e. Distribution of SWC (%) between fertilization and vegetation types related to all replications; c., f. and with the plot average values.

The greatest depletion of SWC was detected within PK treatments across the entire soil profile and under both vegetation types, with depletion also considerable under NPK and NP treatments. Biomass growth was significantly influenced by weeds in treated plots between the BBCH 13 and 19 phenological stages, but water availability did not hamper growth rates in non-fertilized conditions. These findings suggest that, at early stages of crop growth, SWC model simulations need to include better characterisation of depth- and structure-dependent soil water uptake by vegetation.

Read more details in:Lehoczky É., Kamuti, M., Mazsu, N., Sándor, R*. : Changes to soil water content and yield biomass under combined maize and maize-weed vegetation with different fertilization treatments in loam soil. Journal of Hydrology and Hydromechanics,64.