Hi Gerald,

There is substantial research, relating to the influence of plant tannins (in leaf litter) on carbon and nitrogen dynamics in forest soils (see the review available in the link below).  However, there is not much published research on the effect of plant tannins on C & N dynamics in managed pastures.  Perhaps this is due to the lack of ‘plant species diversity’ in managed pastures. After all, many plant secondary compounds (including plant tannins) were thought to act as herbivore deterrents.

Recent focus on ‘tannins in forages’ have been centred on its potential use as feed additives for mitigating enteric CH4 production in ruminants.   Its indirect effect on N partitioning between urine and faecal fractions of excreted N, potentially may have an effect on N2O emissions from pastures, but published research on this area is not yet available.

Kraus, T.E.C. et al. (2003) Tannins in nutrient dynamics of forest ecosystems - a review. Plant and Soil. Volume 256, Issue 1, pp 41-66.

http://link.springer.com/article/10.1023/A%3A1026206511084

Thanks Susantha . I was keen to get information regarding grazed pastures as current research is seeking to get tannins in white clover and ryegrass. RegardsGerald

Hi Gerald I'll send you two pdfs:

Tannins in nutrient dynamics of forest ecosystems - a review

By: Kraus, TEC; Dahlgren, RA; Zasoski, RJ

 PLANT AND SOIL   Volume: 256   Issue: 1   Pages: 41-66   Published: SEP 2003

A review of some important families of refractory macromolecules: Composition, origin, and fate in soils and sediments By: Derenne, S; Largeau, C

SOIL SCIENCE   Volume: 166   Issue: 11   Pages: 833-847   Published: NOV 2001

Tim

Gerald,

Research in this subject is (still) largely focused on developing white clover cultivars with appreciable levels of tannins in its foliage.  If there is any breakthrough, it may be from countries such as New Zealand or Australia where several research teams have been working on this for the past several years.

Here is an abstract of some interesting work : 

In alpine ecosystems, tannin-rich-litter decomposition occurs mainly under snow. With global change, variations in snowfall might affect soil temperature and microbial diversity with biogeochemical consequences on ecosystem processes. However, the relationships linking soil temperature and tannin degradation with soil microorganisms and nutrients fluxes remain poorly understood. Here, we combined biogeochemical and molecular profiling approaches to monitor tannin degradation, nutrient cycling and microbial communities (Bacteria, Crenarcheotes, Fungi) in undisturbed wintertime soil cores exposed to low temperature (0°C/-6°C), amended or not with tannins, extracted from Dryas octopetala. No toxic effect of tannins on microbial populations was found, indicating that they withstand phenolics from alpine vegetation litter. Additionally at -6°C, higher carbon mineralization, higher protocatechuic acid concentration (intermediary metabolite of tannin catabolism), and changes in fungal phylogenetic composition showed that freezing temperatures may select fungi able to degrade D. octopetala’s tannins. In contrast, negative net nitrogen mineralization rates were observed at -6°C possibly due to a more efficient N. immobilization by tannins than N production by microbial activities, and suggesting a decoupling between C and N mineralization. Our results con- firmed tannins and soil temperatures as relevant controls of microbial catabolism which are crucial for alpine ecosystems functioning and carbon storage. source ; Environmental Microbiology (2008) 10(3), 799–809.

Pine litter amended with either tannic acid (TA) or condensed tannins (CTs) was studied to assess the effects on C, N and P mineralization in relation to the fate of tannins by incubation experiments during various time intervals. TA induced a rapid short-term effect resulting in high C respiration and net N and P immobilisation. After one week of incubation, TA was decomposed and net C, N and P mineralization and net nitrification resembled that of the control (non-amended litter). CTs exhibited effects on net mineralization on longer terms, i.e. after several weeks of incubation until the end of the experiment (84 days). While net N and P mineralization were greatly reduced, net nitrification was only slightly affected. Most likely CTs formed complexes with organic N of the substrate thereby reducing net N mineralization, while such complexes were not involved in net nitrification processes. The reduction of net P mineralization is due to the lack of need for P by microbes when they cannot get access to N. The fact that decreasing amounts of extractable CTs were accompanied by increasing effects on mineralization processes with incubation time strongly suggests that CTs were incorporated into the litter in such a way that they were inextricable by the common solvents needed to measure tannins, such as for the Folin–Ciocalteu and HCl–butanol assays.

Source : Nierop, K.G.J., Verstraten, J.M., Tietema, A. et al. Biogeochemistry (2006) 79: 275. doi:10.1007/s10533-005-5274-0