Environmental variables
Precipitation, air temperature, and soil water content data for
2015-2019, years for which we have data of both grass and shrub ANPP,
are shown in Fig. 1. The mean daily air temperature for 2015-2019 was
9°C, and the maximum and minimum temperatures were about 25°C (December
2018) and -9°C (June 2017), respectively (Fig.1A). Annual precipitation
for the period of 2015-2019 ranged from 92 to 233 mm, with 2019 being
the driest and 2017 the wettest year. Between March 29 and April 02,
2017, an extreme precipitation event of 100 mm was recorded (Fig. 1A).
Precipitation in 2013 and 2014, years for which information of grass
productivity is available, was 152.5 and 99.5 mm, respectively. Soil
volumetric water content showed a marked seasonal pattern in the top
soil layer (10 cm), ranging from 5% in summer to 15-20% in winter and
early spring (Fig. 1B). Water addition events were identifiable at that
depth, with transient increases in soil water content. At 40 cm depth,
soil water content had a similar pattern to that of the upper layer, but
water addition events were not visible at that depth (Fig. 1C). The
extreme precipitation event in autumn 2017 resulted in a higher increase
in water content throughout the soil profile compared to other years,
but there was a delay in such increase in deeper soil layers. At 100 and
200 cm soil depth, water content was relatively constant, between 6 and
9% in all treatments except in 2017 (Fig. 1D, 1E). A soil water
retention curve indicated that soil water potential at 40 cm depth was
near 0 MPa when moisture was higher than 12% and was higher than -1 MPa
when moisture was between 9 and 12% (Fig. S3).
Inorganic N and available P content were higher in the +N and +NW
treatments than in the control (Table 2). The addition of nitrogen alone
increased the inorganic N content about three times more than the
co-addition of nutrients and water (p <0.001). Available
P content increased about three times in +N and +NW treatments compared
with the control (p <0.05). Total N and C and N/C ratio
were not significantly different between treatments (Table 2). Soil pH
was lower in the +N and +NW treatments than in the control, decreasing
from 7.15 in the control to 6.40 and 6.48 in those treatments,
respectively (Table 2).