of the former enzymes and consequently reducing N availability. N pool reduction
in soils will require heavier fertilization for agricultural production which in turn
will increase N-footprint related to crop production. If it is true that the increase in
temperature has an immediate effect reducing enzyme activity, it is also true that the
continued effect of climate change has a negative impact on the production of
enzymes by microorganisms compromising enzyme pools and turnover rates. When
enzymes activity is affected by temperature, C degradation will decrease with
consequent reduction on C availability for microorganisms that will slow down N
availability for plant absorption. On the contrary, as mentioned, N-degrading
enzymes will still be active, with cumulative depletion effect on N pool for crop
nutrition.
Diffusion of substrates, enzymes and therefore the products of enzyme activity,
are affected by soil water availability. So, climate change driven drought conditions
will limit diffusion of enzymes and substrates in the soil affecting enzymes contribution to nutrient balances in soils. Indeed, Burns et al. [25] predicted that the
reduction of soil moisture would potentially decrease enzyme activity in response to
a lower microbial biomass and enzyme production. However, the enzyme pool
under drought conditions was stable, which could be explained in two ways: either
mass-specific enzyme production was higher under low water availability, or
enzyme turnover decreased in dry soils, which was the most probable reason to this
observation. The stability of enzymatic processes was not necessarily due to
enzyme activity in situ in dry soils, as reported by the same authors. If this is the
case, N availability is compromised both for protein production for microbes and
for plant nutrition.
Another effect of climate change is the alteration of atmosphere CO 2 content.
This may not directly affect microbial activities because CO 2 concentration in soils
are naturally much higher than in the atmosphere [24]. However, plant direct
responses to elevated CO 2 levels may strongly affect microbial communities due to
the changes in plant's metabolism and processes. These may include increased
water use efficiency, increased exudation of labile C by roots and a faster nutrient
uptake due to a higher plant productivity [25]. Increased rhizodeposition tends to
stimulate microbial biomass, therefore increasing potential enzyme production and
microbial respiration, although [79] did not observe significant effects of high free
air CO 2 presence. CO 2 enrichment is also expected to positively impact nitrogenase
activity and biological N fixation by leguminous plants by (i) increasing plant
mass/N demand and decreasing soil N availability which limits N fixation and
(ii) increased C allocation to root nodules [80, 81]. Whether this is enough to offset
the higher need for N fertilization from the reduced soil N availability, with a
positive impact in the N footprint by enhancing N recycling rather than through the
Haber–Bosch process, remains to be seen.
The greater the knowledge on the degradation of lignin, which constitutes the
recalcitrant fraction of SOM, the more evident is the relationship between C and N
in the enzymatic processes in the soil, since the distribution of potentially
lignin-degrading organisms in soils respond to disturbances associated with
anthropic N deposition and climate change [25]. This leads to the assumption that C
Nitrogen Footprints and the Role …
147
in soils will require heavier fertilization for agricultural production which in turn
will increase N-footprint related to crop production. If it is true that the increase in
temperature has an immediate effect reducing enzyme activity, it is also true that the
continued effect of climate change has a negative impact on the production of
enzymes by microorganisms compromising enzyme pools and turnover rates. When
enzymes activity is affected by temperature, C degradation will decrease with
consequent reduction on C availability for microorganisms that will slow down N
availability for plant absorption. On the contrary, as mentioned, N-degrading
enzymes will still be active, with cumulative depletion effect on N pool for crop
nutrition.
Diffusion of substrates, enzymes and therefore the products of enzyme activity,
are affected by soil water availability. So, climate change driven drought conditions
will limit diffusion of enzymes and substrates in the soil affecting enzymes contribution to nutrient balances in soils. Indeed, Burns et al. [25] predicted that the
reduction of soil moisture would potentially decrease enzyme activity in response to
a lower microbial biomass and enzyme production. However, the enzyme pool
under drought conditions was stable, which could be explained in two ways: either
mass-specific enzyme production was higher under low water availability, or
enzyme turnover decreased in dry soils, which was the most probable reason to this
observation. The stability of enzymatic processes was not necessarily due to
enzyme activity in situ in dry soils, as reported by the same authors. If this is the
case, N availability is compromised both for protein production for microbes and
for plant nutrition.
Another effect of climate change is the alteration of atmosphere CO 2 content.
This may not directly affect microbial activities because CO 2 concentration in soils
are naturally much higher than in the atmosphere [24]. However, plant direct
responses to elevated CO 2 levels may strongly affect microbial communities due to
the changes in plant's metabolism and processes. These may include increased
water use efficiency, increased exudation of labile C by roots and a faster nutrient
uptake due to a higher plant productivity [25]. Increased rhizodeposition tends to
stimulate microbial biomass, therefore increasing potential enzyme production and
microbial respiration, although [79] did not observe significant effects of high free
air CO 2 presence. CO 2 enrichment is also expected to positively impact nitrogenase
activity and biological N fixation by leguminous plants by (i) increasing plant
mass/N demand and decreasing soil N availability which limits N fixation and
(ii) increased C allocation to root nodules [80, 81]. Whether this is enough to offset
the higher need for N fertilization from the reduced soil N availability, with a
positive impact in the N footprint by enhancing N recycling rather than through the
Haber–Bosch process, remains to be seen.
The greater the knowledge on the degradation of lignin, which constitutes the
recalcitrant fraction of SOM, the more evident is the relationship between C and N
in the enzymatic processes in the soil, since the distribution of potentially
lignin-degrading organisms in soils respond to disturbances associated with
anthropic N deposition and climate change [25]. This leads to the assumption that C
Nitrogen Footprints and the Role …
147