Timing of drought in the growing season

Experimental setup:

For the experiment we established six grasses in monoculture that are commonly used in agricultural practice in August 2013 on 168 plots (3 × 5 m). This timing is following best practice and guarantees full establishment of the sward (including vernalisation during winter) and full productivity in the following year. The six grasses established were Lolium perenne L. early flowering (LPe; cultivar ‘Artesia’), Lolium perenne L. late flowering (LPl; cultivar ‘Elgon’), Dactylis glomerata L. early flowering (DGe; cultivar ‘Barexcel’), Dactylis glomerata L. late flowering (DGl; cultivar ‘Beluga’), Lolium multiflorum Lam. var italicum Beck (LM; cultivar ‘Midas’), and Poa pratensis L. (PP; cultivar ‘Lato’). Phosphorous, potassium and manganese were applied following national fertilization recommendations for intensely managed grasslands at the beginning of each growing season (39 kg P/ha, 228 kg K/ha, 35 kg Mg/ha). In addition, all plots received the same amount of mineral N fertilizer as ammonium-nitrate (280 kg N/ha, divided into six applications per year). The solid N fertilizer was applied at the beginning of the growing season (80 kg N/ha) and after each of the first five cuts (40 kg N/ha each time).

Each of the six grasses was subject to four treatments: one rain-fed control and three seasonal drought treatments (spring, summer, fall). A drought treatment lasted for ten weeks. Drought was simulated using rainout shelters that excluded rainfall completely on the treatment plots. The rainout shelters were tunnel-shaped and consisted of steel frames (3 × 5.5 m, height: 140 cm) that were covered with transparent and UV radiation transmissible greenhouse foil (Lumisol clear, 200 my, Hortuna AG, Winikon, Switzerland). To allow air circulation, shelters were open on both opposing short ends and had ventilation openings of 35 cm height over the entire length at the top and the bottom at both long sides. Rain-fed controls were subject to the natural precipitation regime. However, when soil water potential sank below -0.5 MPa due to naturally dry conditions, control plots were additionally watered with 20 mm of water (300 l per plot). Watering happened once on June 16th and 17th 2014 and three times in 2015 (7.7., 14.7., 11.8.).

Relative humidity and air temperature were measured hourly at the field site using VP-3 humidity, temperature and vapor pressure sensors (Decagon Devices, Inc., Pullman, WA, USA). Measurements were conducted in control and treatment plots under the rainout shelters. Information on precipitation and evapotranspiration was provided by the national meteorological service stations that were in close proximity of our research site (average of the two surrounding meteorological stations Zurich Affoltern in 1.4 km distance and Zurich Kloten in 4.5 km distance). Soil water potential was measured in 10 cm depth on an hourly basis using 32 MPS-2 dielectric water potential sensors (Decagon Devices, Inc., Pullman, WA, USA). The soil water potential sensors were evenly distributed over the field and treatments. Daily means of all measurements were calculated per treatment, but across grasses since no grass-specific alterations in soil water potential were expected or measured.

Aboveground biomass was harvested six times per year at a five-week interval in 2014 and 2015, and once in spring 2016. The harvests were synchronized with the drought treatments and occurred five and ten weeks after the installation of the shelters on a respective treatment. For the harvest, aboveground biomass was cut at 7 cm height above the ground and harvested from a central strip (5 × 1.5 m) of the plot using an experimental plot harvester (Hege 212, Wintersteiger AG, Ried/I., Austria). The fresh weight of the total harvest of a plot was determined with an integrated balance directly on the plot harvester. Dry biomass production was determined by assessing dry weight – fresh weight ratios of the harvested biomass. For this a biomass subsample was collected for each plot and the fresh and dry weight (dried at 60°C for 48 h) were determined.

Belowground biomass of four grasses (DGe, DGl, LPe and LPl) was harvested six times per year, at the end of each drought period and six to eight weeks after drought release, from the respective treatment and control plots using a manual soil auger with a diameter of 7 cm. For each plot samples of the upper 14 cm soil were taken from two different spots (one sample directly from a tussock and one from in between tussocks) and pooled as one sample per plot. All samples were washed using a sieve with a mesh size of 0.5 cm × 0.5 cm and weighed after drying (at 60°C for 72 h).

In order to allow the comparison of grassland productivity in the different treatments across the two years we standardized the productivity that occurred in between two harvest periods (i.e. during five weeks) for growth related temperature effects and calculated temperature-weighted growth rates for each of the six grasses (DMYTsum). For this purpose, we determined temperature sums of daily mean air temperature above a base temperature of 5°C (Tsum) for each growth period (i.e. 5 weeks prior to harvest in the unsheltered control as well as the sheltered treatment plots). Dry matter yield (DMY) of a given harvest was then divided by the temperature sum of the corresponding time period to obtain temperature-weighted growth rates (henceforth referred to simple as growth rate):

DMYTsum = DMY(g/m2)/Tsum(°C).

To determine the absolute change of growth (ACG) of a drought treatment on aboveground growth rate we calculated the difference between temperature-weighted growth rates in a drought treatment (drt) and the corresponding control (ctr):

ACG = DMYTsum(drt)-DMYTsum(ctr).

To determine the relative change of growth (RCG) due to drought, we calculated percentage change of temperature-weighted growth rates:

RCG = 100×(DMYTsum(drt)/DMYTsum(ctr)-1).

Annual aboveground NPP as an average of the different grasses was determined by adding up the dry matter yields of the six harvests of a growing season. These data were not temperature-corrected (DMY).

Used Instruments:

• VP-3 humidity, temperature and vapor pressure sensors (Decagon Devices, Inc., Pullman, WA, USA)
• MPS-2 dielectric water potential sensors (Decagon Devices, Inc., Pullman, WA, USA) experimental plot harvester (Hege 212, Wintersteiger AG, Ried/I., Austria)
• manual soil auger with a diameter of 7 cm

Data

List of parameters measured or described:

• soil water potential
• air temperature
• vapor pressure deficit
• precipitation
• phenological stage (seperated into vegetative and generative)
• fresh weight
• dry weight
• δ¹³C of plant material
• nitrogen content of harvested plant material
• digestible organic matter of plant harvested material
• crude protein content of plant harvested material
• crude ash content of plant harvested material

Measuring periods (date, daytime):

The data was collected from March 12th 2014 until May 3rd 2016

Column descriptions and units:

• Plot: plot number
• Cultivar: short key for tested cultivar monoculture
  • LPf: Lolium perenne L. 'Artesia' (early-flowering cultivar)
  • LPs: Lolium perenne L. 'Elgon' (early-flowering cultivar)
  • DGf: Dactylis glomerata L. 'Barexcel' (early-flowering cultivar)
  • DGs: Dactylis glomerata L. 'Beluga' (early-flowering cultivar)
  • LM: Lolium multiflorum Lam. var italicum Beck 'Midas'
  • PP: Poa pratensis L. 'Lato'
  • TR: Trifolium repens L. 'Bombus'
• Treatment: short key for the implemented kind of seasonal drought;
  • F: spring drought
  • S: summer drought
  • H: autumn drought, K: control
• Serie: type of serie (A or B)
• Rep: number of replication in the experiment
• Row: number of row in the field: 1-5
• Bock: position of plots in the field: block 1 is on the westernmost site of the field, block 7 on the easternmost site
• Recovery: short key for:
  • C - control
  • bT - before treatment conditions
  • T - treatment conditions;
  • R - recovery conditions (after treatment)
• Harvest: number of harvest
• Date: date of harvest
• DOY: date of harvest as day of year
• FWplot: fresh weight per plot (kg/plot)
• FWha: fresh weight (kg/ha)
• percentageDW: percentage dry weight (%)
• DW: dry weight (kg/ha)
• Growthdays: number of days in growing period between two harvests
• DWdays: rate of growth (calculated by: DW (kg/ha)/Growthdays) (kg/ha/day)
• ATsum: sum of daily air temperature averages above 5°C from previous harvest until current harvest (°C)
• RadSum: sum of daily radiation averages from previous harvest until current harvest (W/m2); Daily radiation averages as a mean of MeteoSwiss stations Zürich-Affoltern and Zürich-Kloten; RadSum of current treatment reduced by 10%, because of radiation-reducing effect of rainout shelters (see Hortuna AG)
• VPDsum: sum of daily vapour pressure deficit (VPD) averages (derived from daily air temperature and daily relative humidity) from previous harvest until current harvest (kPa)
• RainSum: sum of daily rainfall data from previous harvest until current harvest (mm); Daily rainfall data as a mean of MeteoSwiss stations Zürich-Affoltern and Zürich-Kloten; manual watering events added to control plots; RainSum of current treatment set to 0, because of rainout shelters excluding the rain
• SWP: soil water potential (MPa)
• medianSWP10: median of daily mean soil water potential in 10cm depth from previous harvest until current harvest (MPa)
• cumSWP10: cumulative soil water potential in 10cm depth from previous harvest until current harvest (MPa)
• SWPdays10: number of days within the growing period (between two harvests) with a soil water potential in 10 cm depth of or below -1.5MPa
• medianSWP30: median of daily mean soil water potential in 30cm depth from previous harvest until current harvest (MPa)
• cumSWP30: cumulative soil water potential in 30cm depth from previous harvest until current harvest (MPa)
• SWPdays30: number of days within the growing period (between two harvests) with a soil water potential in 30 cm depth of or below -1.5MPa
• Phen: phenological stage right before harvest/at garvest date (0: vegetative; 1: generative)
• IsoC: drift corrected δ¹³C value (‰)
• IsoN: drift corrected δ15N value (‰)
• N: nitrogen concentration (g/kg DW)
• VOS: digestibale organic matter (g/kg DW)
• RP: crude protein (g/kg DW)
• RA: crude ash (g/kg DW)
• Nc: critical plant nitrogen concentration, corresponding to the observed crop mass (DW); calculated after Lemaire 1997: Diagnosis of nitrogen uptake in crops
• NNI: nitrogen nutrition index (N/Nc); Ratio of actual plant nitrogen concentration and critial plant nitrogen concentration; see Lemaire 1997: Diagnosis of nitrogen uptake in crops

List of archived files:

• Biomass:
  • 181004_Biomass_2014
  • 181004_Biomass_2015
  • 181004_Biomass_2016
  • 181004_RootBiomass_2014
• Phenology:
  • 181004_Phenology_2014
  • 181004_Phenology_2015
• Physiology:
  • 181004_Isotopes_201415
  • 181004_PlantWaterPotential
  • 181004_PredawnPlantWaterPotential
  • 181004_StomatalConductance
• Soil Moisture:
  • 181004_Soil sensors_daily mean_2014
  • 181004_Soil sensors_daily mean_2015