Literature review: Impact of Chilean needle grass ... - Weeds Australia

Nitrogen dynamics of competing C 3 and C 4 grasses
C 4 grasses are superior competitors in undistrurbed grassland because of they way they use and sequester N and extraneous
addition of soil N benefits alien annual grasses and decreases species richness of native flora that evolved under conditions of
low soil N (Milton 2004).
Moore (1973) suggested that the success of T. triandra as a warm-season C 4 grass in southern latitudes is due to an ability to
sequester N and other nutrients as they are mineralised. Short term increases in N mineralisation after fire in ungrazed T.
triandra grassland therefore advantage the dominant grass. N appears to be the key nutrient determining the balance between C 4
and C 3 grasses (Wedin 1999, Groves and Whalley 2002, Groves et al. 2003a). Soils are more commonly deficient in it than any
other nutrient, and plants growing in N-deficient soils usually have a high root-shoot ratio (Salisburyand Ross 1992). Plant
growth is believed to be primarily limited by the availability of N in the soil (Eschen et al. 2007). Unlike other nutrients, N has
little interaction with inorganic soil minerals and its plant-availability is almost totally regulated by biotic processes (Wedin
1999). The amount of plant-available soil N is determined by mineralisation from organic matter by soil microbes,
immobilisation in microbial and plant biomass, deposition (external fertilisation) and external losses (denitrification and
leaching) (Moretto and Distel 2002). Soil inorganic nitrogen content largely reflects the balance between mineralization of N and
immobilisation of N by soil microbial biomass (Andrioli and Distel 2008). Most soil N is contained in the humus, locked in C-N
bonds that are energetically very expensive for decomposer organisms to break (Wedin 1999). High lignin concentrations and
high C:N ratios in plant litter commonly immobilise the N pool and result in low available N in the soil (Moretto and Distel
2002). C 4 grasses use N more efficiently than C 3 grasses (Monson 1989, Bouchenak-Khelladi et al. 2009) and therefore have
roots and leaves with higher C:N ratios and litter of a lower quality. For example the ratio in Schyzachyrium scoparium (Michx.)
Nash (C 4 ) roots was 100 and leaf litter 110 (Andrioli and Distel 2008) compared to roots c. 39-59 and leaf litter c. 40-60 in four
Nassella spp. assessed (Andrioli and Distel 2008). T. triandra, as a C 4 species, produces a relatively high amount of biomass, low
in protein, compared to co-occurring native C 3 grasses (Moore 1993, Nie et al. 2009), and its litter has a high C:N ratio, so it can
be conceived of as a climax species in grassland succession, where the climax is characterised by a stable species assemblage,
with low levels of available soil N and relatively high biomass (Moore 1993).
Higher quality plant litter decomposes more rapidly and results in net mineralisation of N, whereas lower quality litter
decomposes more slowly and results in net immobilisation of N (Andrioli and Distel 2008). Andrioli and Distel (2008) found
little difference between the litter quality of several Nassella spp. (not including N. neesiana) in semiarid (mean annual rainfall
400 mm) Argentina and no differences in their influence on soil inorganic N content or potential N mineralisation. Variation
measured within the Nassella spp. was low in comparison to C 4 grasses. Presumably N. neesiana has similar higher quality litter,
that markedly differs in its biodegradibiltiy to the low quality T. triandra litter.
Mineralisation of soil N in temperate Australian takes place during the summer, reaching a peak at the end of summer (Moore
1993). The nitrate content of the top 10 cm of soil under T. triandra grassland at the end of summer has been found to not exceed
5 ppm, in comparison to >36 ppm under Austrodanthonia C 3 grasses (Moore 1993). Soils under the cool season native grasses
therefore had large pools of labile N when the winter growing season commenced, facilitating invasion by cool-season annuals,
while intact T. triandra was resistant to invasion: “By growing when mineralisation processes are actually or potentially active,
and utilising or otherwise limiting the accumulation of labile nitrogen in the soil surface, Themeda, seemingly, gives stability to
the grassland community” (Moore 1993 p. 352).
When the C:N ratio of grass litter is more than 30:1, the rate of decomposition of the litter is slow, microbial decomposers are N-
limited, N is largely immobilised, and there is little or no release of nitrate and ammonium into soil solution (Wedin 1999,
Groves and Whalley 2002, Moretto and Distel 2002). In such situations with a low rate of N mineralistation, species with a high
N use efficiency, usually the C 4 grasses, have a competitive advantage, and are able to efficiently deplete the low soil nitrate
pools. The C:N ratio of litter and roots of such grasses is generally much greater than 30:1, and they immobilise the N, and
buffer against N pulses created by disturbance, so perpetuate their own dominance (Wedin 1999, Groves and Whalley 2002). T.
triandra is a more efficient user of N than N. neesiana, and healthy T. triandra stands lock up system pools of N in high-C
biomass. This array of N-related mechanisms explains the resistance to invasion by exotic species of T. triandra grasslands
reported by Hocking (1998) and Wijesuriya and Hocking (1999).
When disturbance results in the death of T. triandra, levels of available N are increased and this results in changes in the floristic
composition of the grassland (Moore 1993). Disturbances including continuous grazing, fertiliser addition and cultivation, result
in increased rates of N mineralisation and higher soil nitrate and ammonium levels (Wijesuriya and Hocking 1999, Groves and
Whalley 2002). Soil disturbance involving digging and homogenisation of soil of T. triandra grassland thus results in a major
increase in the above-ground biomass of dicot weeds and annual grasses. Wijesuriya and Hocking (1999) found that
approximately 90% of a total of 60 kg ha -1 dry weight above ground in late spring, 70 days after such disturbance, consisted of
exotic annual grasses, “thistles” and “flat weeds”. This reflected a simultaneous nearly ten-fold increase in the amount of plantavailable
soil N and an approximate doubling of available soil P. Addition of c. 25 kg ha -1 of both N and P fertiliser, combined
with digging and homogenisation of the soil, produced total above ground biomass over the same period of c. 150 kg ha -1 dry
weight, of which approximately 90% again consisted of these exotic weeds (Wijesuriya and Hocking 1999).
These authors demonstrated that addition of N and P to T. triandra grassland at rates of c. 25 kg of N or P ha -1 resulted in weed
flushes and that cultivation resulted in rapid mineralisation of organic matter and a consequent ‘pulse’ of N. In experiments on
basaltic clays at Derrimut, Victoria, after 14 days, the rate of mineralisation of N in soil dug out, homegenised to very small
particle size and replaced in plots was 4.7 times that of undisturbed soil, while the rate for P was not significantly different to
undisturbed soil. Total available soil N continued to increase in the disturbed soil for 70 days, at which time it was
approximately 10 times that of undisturbed soil. Available P was also significantly higher in the dug soil after 70 days. These
authors also compared plots in which there was no soil disturbance with those in which soil was dug out, homogenised and
returned, and subsequently unfertilised, fertilised with N, P, both N and P, or treated with sucrose (as a C source). In early
summer, 70 days after disturbance, >95% of plant biomass in disturbed plots consisted of exotic weeds, mainly annual
Asteraceae and annual Poaceae, with the Poaceae accounting for about half the biomass of the dicotyledonous species. Undug
plots carried >90% T. triandra. Species diversity was similar in plots dug and treated with N and P, or both N and P, but annual
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